Log-structured distributed storage using a single log sequence number space

ABSTRACT

A distributed database system may implement log-structured distributed storage using a single log sequence number space. A log for a data volume may be maintained in a log-structured distributed storage system. The log may be segmented across multiple protection groups according to a partitioning of user data for the data volume. Updates to the log may be assigned a log sequence number from a log sequence number space for the data volume. A protection group may be determined for an update according to which partition of user data space the update pertains. Metadata to be included with the log record may indicate a previous log sequence number of a log record maintained at the protection group. The log record may be sent to the protection group and identified as committed based on acknowledgments received from storage nodes implementing the protection group.

This application is a continuation of U.S. patent application Ser. No.14/036,775, filed Sep. 25, 2013, now U.S. Pat. No. 9,552,242, which ishereby incorporated by reference herein in its entirety.

BACKGROUND

Data storage systems have implemented many different storage schemes forefficiently and reliability persisting data. Storage schemes implementedon a distributed system architecture are often deployed when storagesystem client applications, such as database systems, require greateravailability of the data persisted in the data storage system. Commonsolutions to making data available including storing one or moreversions or replicas of data on multiple storage nodes. However, byincreasing the number of versions or replicas, the complexity andoperational costs for maintaining a consistent view of persisted dataincreases. Synchronization protocols are used to ensure consistency whenchanges are made to the versions or replicas of data across the storagenode. However, typical synchronization protocols often increase the timeand resources required to perform the change consistently. The costs tomaintain a consistent view in the data storage system can dull the veryadvantages of implementing the distributed storage system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating log-structured distributedstorage using a single log sequence number space, according to someembodiments.

FIG. 2 is a block diagram illustrating a service system architecturethat may be configured to implement a network-based database service anda network-based distributed storage service, according to someembodiments.

FIG. 3 is a block diagram illustrating various components of a databasesystem that includes a database engine and a separate distributedstorage service, according to some embodiments.

FIG. 4 is a block diagram illustrating a distributed storage system,according to some embodiments.

FIG. 5 is a block diagram illustrating the use of a separate distributedstorage system in a database system, according to some embodiments.

FIG. 6 is a block diagram illustrating how data and metadata may bestored on a storage node of a distributed storage system, according tosome embodiments.

FIG. 7 is a block diagram illustrating an example configuration of adatabase volume, according to some embodiments.

FIG. 8 is a high-level flowchart illustrating a technique forimplementing log-structured distributed storage using a single logsequence number space, according to some embodiments.

FIG. 9 is a high-level flowchart illustrating a technique for recoveryin log-structured distributed storage using a single log sequence numberspace, according to some embodiments.

FIG. 10 is a high-level flowchart illustrating a technique for selectingcandidate log records for log recovery, according to some embodiments.

FIG. 11 is a block diagram illustrating a log segmented according topartitions of user data space in log-structured distributed storage,according to some embodiments.

FIG. 12 is an example computer system, according to various embodiments.

While embodiments are described herein by way of example for severalembodiments and illustrative drawings, those skilled in the art willrecognize that the embodiments are not limited to the embodiments ordrawings described. It should be understood, that the drawings anddetailed description thereto are not intended to limit embodiments tothe particular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope as defined by the appended claims. The headings usedherein are for organizational purposes only and are not meant to be usedto limit the scope of the description or the claims. As used throughoutthis application, the word “may” is used in a permissive sense (i.e.,meaning having the potential to), rather than the mandatory sense (i.e.,meaning must). The words “include,” “including,” and “includes” indicateopen-ended relationships and therefore mean including, but not limitedto. Similarly, the words “have,” “having,” and “has” also indicateopen-ended relationships, and thus mean having, but not limited to. Theterms “first,” “second,” “third,” and so forth as used herein are usedas labels for nouns that they precede, and do not imply any type ofordering (e.g., spatial, temporal, logical, etc.) unless such anordering is otherwise explicitly indicated.

Various components may be described as “configured to” perform a task ortasks. In such contexts, “configured to” is a broad recitation generallymeaning “having structure that” performs the task or tasks duringoperation. As such, the component can be configured to perform the taskeven when the component is not currently performing that task (e.g., acomputer system may be configured to perform operations even when theoperations are not currently being performed). In some contexts,“configured to” may be a broad recitation of structure generally meaning“having circuitry that” performs the task or tasks during operation. Assuch, the component can be configured to perform the task even when thecomponent is not currently on. In general, the circuitry that forms thestructure corresponding to “configured to” may include hardwarecircuits.

Various components may be described as performing a task or tasks, forconvenience in the description. Such descriptions should be interpretedas including the phrase “configured to.” Reciting a component that isconfigured to perform one or more tasks is expressly intended not toinvoke 35 U.S.C. §112, paragraph six, interpretation for that component.

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

DETAILED DESCRIPTION

Various embodiments of log-structured distributed storage using a singlelog sequence space are described herein. A log-structured distributedstorage system may segment log storing updates for storage client datavolume across multiple protection groups. These protection groups may beimplemented by multiple storage nodes providing redundant storage ofportions of the log and user data space of the data volume partitionedamong the protection groups. Distributed storage increases dataavailability for access requests from a storage system client. Readrequests, for example, may be made to one or more of the storage nodesin order to read data. Write requests, or updates, to the data may alsobe made. These write requests may need to be made consistent across thestorage nodes storing the data so that read requests to differentstorage nodes do not return stale or erroneous data.

In at least some embodiments, log-structured distributed storage mayprovide consistent views of a data volume stored for a storage client.Different partitions of a user data space of the data volume may bestored among different protection groups. The log storing updates to theuser data may be segmented according to this partitioning of user dataspace. A log segment pertaining to a partition of user data may beco-located with the partition of the user data at a same protectiongroup. As new updates or changes are added to the log, a log recordindicating the update or change may be sent to the protection groupwhich maintains the partition of user data to which the update pertains.The log record may be maintained at the protection group along withother log records segmented to the protection group also correspondingto changes pertaining to the partition of user data at the protectiongroup. Thus, when performing access or other operations with regard to apartition of user data, log records for the partition may already resideat the protection group sufficient to perform the operation (e.g.,generate a current or prior version of a user data page for a readrequest).

Although the log is segmented across different protection groupsaccording to the respective partitions of the user data of the datavolume maintained at each protection group, consistency of the log maybe maintained across the log by using a single log sequence numberspace, in various embodiments. As log records indicating updates orchanges are generated, a log sequence number from a common log sequencenumber space may be assigned to the log record. A log sequence numberspace may, in some embodiments, be a monotonically increasing numberspace. Assignments from the number space may be sparse (e.g., 1, 7, 16,23, 25, etc . . . ) in some embodiments, contiguous (e.g., 1, 2, 3, 4,5, etc . . . ) in some embodiments, or some combination of both.Metadata may also be included with log records that indicates a previouslog sequence number of a log record maintained at the same protectiongroup. In some embodiments, metadata may also indicate a previous logsequence number in the log sequence number space which may be maintainedat any one of the protection groups across which the log is segmented.

Metadata included with the log record may be such that a position in arecovery sequence for the protection group and the log sequence numberspace may be identifiable. A recovery sequence for each protection groupmay allow each protection group to be independently recoverable, withoutreference to another protection group. However, using the log sequencenumber space, dependencies across the multiple protection groups betweendata pages (e.g., such as changes made to different portions of userdata as part of a user transaction in a database system) may be keptconsistent. Thus, distributed log-structured storage may reduce thelatency in operations to consistently persist updates to the data volumein the log, as well as reducing overhead operational costs. Log recordsmay be identified as committed to the log for the entire data volume,without being persisted among multiple protection groups or performing atwo-phase commit, in some embodiments.

Typically, consistency mechanisms for distributed systems, such asdistributed storage systems, include one or more different steps thatare performed during the workflow to make a change or perform anoperation at a distributed storage system. For instance, one commonsynchronization method is a two-phase commit. A two-phase commit,typically implements a transaction coordinator system by first, sendinga commit request message to nodes in the distributed system. Each nodemust then respond back with an agreement message to commit. Onceagreement is received from all of the storage nodes, a commit message toperform the requested operation or make the requested change is sent toeach node, and then each node must respond with an acknowledgment.Finally, upon receiving an acknowledgment from all of the nodes, theoperation or change may be considered durable and consistent (i.e.committed) across the distributed system. Other synchronizationprotocols introduce similar or more complicated synchronization schemes,such as the Paxos algorithm or three-phase commit protocol. While eachof these, and other synchronization techniques, ensure consistency in adistributed system, devices, systems, applications or other clients of adistributed system that perform operations or make changes in thedistributed storage system may have to perform many different steps,utilizing more system resources and increasing latency for performingoperations.

In an alternative method for ensuring consistency, some typical systemsdistribute log records among different nodes or groups of nodes in adistributed system. However, these systems send log records to storagenodes according to time, such as by striping log records acrossdifferent storage nodes or storing ranges of log sequence numberstogether. When providing access to data described by logs distributed inthis way, the entire log may still have to be read to perform access orother operations (such as recovery), increasing latency for performingoperations.

In various embodiments, log-structured distributed storage using asingle log sequence number space may ensure consistent views of a datavolume stored for a storage client while reducing latency andoperational costs for committing changes to the data volume. In variousembodiments, updates or changes to the data volume stored atlog-structured distributed storage may be committed to the log for thedata volume when committed to a particular protection group to which thepartition of user data space the update pertains. As noted above, logrecords may be sent to a protection group with metadata indicating aprevious log sequence number of a log record maintained at theprotection group. In this way, each protection group may, in at leastsome embodiments, be independently recoverable by evaluating thismetadata for log records maintained at the protection group. FIG. 1 is ablock diagram illustrating log-structured distributed storage using asingle log sequence space, according to some embodiments.

A storage client 100 may utilize log-structured distributed storagesystem 120 to store data volume 110 for storage client 100, or anothersystem, device, or application that is itself a client of storage client100. Storage client 100 may be one of various systems, applications, ordevices configured to access data volume 110. In at least someembodiments, storage client 100 may be a database system, such as adistributed database system. In order to update data volume 110, storageclient 100 may send log records 104 indicating changes to log-structureddistributed storage system 120. Different types of log records may besent that may indicate changes to data and/or metadata at thelog-structured data store. For example, for a storage client that is adatabase the different types of log records may include redo records,undo records, transaction table entries, etc . . . . In at least someembodiments, the single log sequence number space for the log-structureddistributed storage may be the same log sequence number space fordatabase log records (when the storage client is a database system),such as those used to implement write-ahead logging and other databasetechniques. As various log record types for log-structured data storesare well-known to those of ordinary skill in the art, the previousexamples are not intended to be limiting.

As noted above, a log 102 for data volume 110 may be maintained. Fromstorage client's 100 perspective and/or the perspective of a client ofstorage client 100, log 102 may appear to be a single log 102 indicatingupdates to storage client user data 122. Log records in log 102 may havelog sequence numbers from a log sequence number space, and may beordered according to the log sequence numbers in the log sequence numberspace. FIG. 1 illustrates, for instance, LSN 1 occurring before LSN 2,LSN 2 occurring before LSN 3, and so on for log records LSN 1-LSN 22. Insome embodiments, updates to log 102 may processed asynchronously.Therefore a log record 104 may be sent be included in log 102, and thena next log record 104 may be sent to be included in log 102, beforereceiving an acknowledgment that the first log record has been committedto the log. Thus, log records in some embodiments some log records mayarrive before or be committed before other log records, hence theillustrated gaps between LSN 17, LSN 20, and LSN 22.

While storage client's 100 view of data volume 110 may appear to be asingle log 102 and user data 122, log-structured distributed storagesystem 120 may segment the log across multiple protection groups. Forexample, FIG. 1 illustrates 3 protection groups, 120 a, 120 b, and 120c, which store different segments of the log 102, such as log segment102 a, 102 b, and 102 c. Each protection group may be implemented by aplurality of storage nodes 118, systems, or devices, such as computingsystem 1200 described with regard to FIG. 12 below. Also maintained ateach protection group 120 a-120 c, is a user data partition, such asuser data partitions 122 a, 122 b, and 122 c. A user data partition maybe a range of user data 122, such as a byte range, page range, etc.However, various other partitioning arrangements of user data 122 may beenvisioned, and therefore, the previous examples are not intended to belimiting.

FIG. 1 illustrates that partition specific log records may be sent toand maintained at specific protection groups. Log records 104 aindicating updates to user data partition 122 a may be sent toprotection group 120 a to be maintained. Likewise, log records 104 b,and 104 c indicating updates to user data partition 122 b and 122 crespectively may be sent to protection group 120 b and 120 c. Thus, insome embodiments, in response to receiving an update to log 102, aprotection group according to which the update pertains may bedetermined. Metadata may be generated to be included with the logrecord, in at least some embodiments, that indicates a log sequencenumber of a previous log record maintained at the determined protectiongroup such that a position in a recovery sequence for the protectiongroup and a position in the log sequence number space are identifiable.The log record, and attendant metadata, may then be sent to thedetermined protection group, as illustrated in FIG. 1. In someembodiments, based, at least in part, on acknowledgments received fromstorage nodes 118 implementing a protection group to which the logrecord is sent, the log record may be identified as eligible fordurability to log 102.

Segmenting the log records of log 102 in this way my lead to twodifferent observations. First, each protection group may have its ownrespective log, with an ordering that is not identifiable based on theLSN of a log record alone. For example, log segment 102 a stores logrecord LSN 2, LSN 3, LSN 4, and LSN 8. LSNs 5, 6, 7 are located on otherprotection groups, so protection group 120 a may need to determinewhether or not it is missing a log record (such as when participating inrecovery). Thus, metadata, either maintained in the log records orcollectively somewhere else in the protection group, may indicateprevious log sequence numbers for each log record. In this way, arecovery sequence or ordering for log records may be identifiable sothat missing log records may be determined. Moreover, access operationsfor data in the user data partition may be performed at the protectiongroup without a need to obtain log records in the log for the datavolume from other protection groups. Metadata for LSN 8 indicates, forinstance, that LSN 4 is the previous log record to LSN 8 at protectiongroup 120 a (and thus no gap is illustrated). LSN 20, however, mayindicate that LSN 18 is the previous log record, which has not beenpersisted at protection group 120 a (thus a gap is illustrated).

A second observation, log records that are dependent on another logrecord (e.g., as part of a system or user transaction in a databasesystem) may be maintained at different protection groups. If, forinstance, LSN 16 is dependent on LSN 15 in order to indicate changes touser data 122, protection group 120 a may be unaware of whether or notLSN 15 is actually committed to log 102. In some embodiments, metadatamaintained for each log record may indicate these dependencies on otherlog records, such as by indicating a previous LSN of a log record in thelog sequence number space.

In addition to these observations, it may also be noted that log recordsmay be segmented across the protection groups without striping the logrecords, or storing the log records according to different ranges ateach protection group. For example, each log segment 102 a, 102 b, and102, stores at least one log record with an LSN between the ranges of 6and 15.

An effect of segmenting log records across protection groups accordingto user data partitions is that each protection group may be recoveredindependently, without reference to other protection groups. Recoverymay also be performed in parallel or near parallel at each protectiongroup. Recovery may mean determining for a storage client that hasfailed and restarted (or a new storage client replacing an old/failedstorage client) the committed state of data volume 110. Part of recoverymay involve determining a recovery point in the log 102 for data volume110. The recovery point may be the last committed log recordacknowledged to client 100 with no missing or incomplete log records, ora last durably persisted log record (which may not have been identifiedas committed to client 100 even though it is durably persisted),indicating a consistent view of the data volume 110 from which theclient 100 may continue to operate. LSN 17 is an example of a committedlog record that is complete with no missing prior log records upon whichLSN 17 (or another log record) may be dependent. As part of determiningthe recovery point for log 102, each protection group may be able todetermine a completion point for log records in the log segmentmaintained at the protection group based on metadata (discussed above)maintained for the log records indicating the previous LSN of a logrecord at the protection group. Candidate log records for the recoverypoint may be determined at protection groups according to the completionpoint and/or metadata maintained for the log records and sent to arecovery coordinator or client to determine the log recovery point.FIGS. 9-11 discuss related techniques to implement recovery for storageclient 110 in greater detail below, and thus, the previous discussion isnot intended to be limiting.

Please note, FIG. 1 is provided as a logical illustration of alog-structured distributed storage system using a single log sequencenumber space, and is not intended to be limiting as to the physicalarrangement, size, or number of components, modules, or devices,implementing a distributed storage system. For example, the number ofprotection groups may vary. Different numbers of storage nodes ordevices implementing protection groups may change, as well as therespective partitions of user data, segmentations/orderings of log etc.

The specification first describes an example of a log-structureddistributed storage using a single log sequence number space system,according to various embodiments. The example log-structured distributedstorage service may store data for many different types of clients, invarious embodiments. One such client may be a network-based databaseservice, described in further detail below. Included in the descriptionof the example network-based database service are various aspects of theexample network-based database service along with the variousinteractions between the database service and the log-structureddistributed storage service. The specification then describes aflowchart of various embodiments of methods for implementinglog-structured distributed storage using a single log sequence space.Next, the specification describes an example system that may implementthe disclosed techniques. Various examples are provided throughout thespecification.

The systems described herein may, in some embodiments, implement anetwork-based service that enables clients (e.g., subscribers) tooperate a data storage system in a cloud computing environment. In someembodiments, the data storage system may be an enterprise-class databasesystem that is highly scalable and extensible. In some embodiments,queries may be directed to database storage that is distributed acrossmultiple physical resources, and the database system may be scaled up ordown on an as needed basis. The database system may work effectivelywith database schemas of various types and/or organizations, indifferent embodiments. In some embodiments, clients/subscribers maysubmit queries in a number of ways, e.g., interactively via an SQLinterface to the database system. In other embodiments, externalapplications and programs may submit queries using Open DatabaseConnectivity (ODBC) and/or Java Database Connectivity (JDBC) driverinterfaces to the database system.

More specifically, the systems described herein may, in someembodiments, implement a service-oriented architecture in which variousfunctional components of a single database system are intrinsicallydistributed. For example, rather than lashing together multiple completeand monolithic database instances (each of which may include extraneousfunctionality, such as an application server, search functionality, orother functionality beyond that required to provide the core functionsof a database), these systems may organize the basic operations of adatabase (e.g., query processing, transaction management, caching andstorage) into tiers that may be individually and independently scalable.For example, in some embodiments, each database instance in the systemsdescribed herein may include a database tier (which may include a singledatabase engine head node and a client-side storage system driver), anda separate, distributed storage system (which may include multiplestorage nodes that collectively perform some of the operationstraditionally performed in the database tier of existing systems).

As described in more detail herein, in some embodiments, some of thelowest level operations of a database, (e.g., backup, restore, snapshot,recovery, log record manipulation, and/or various space managementoperations) may be offloaded from the database engine to the storagelayer (or tier), such as a distributed storage system, and distributedacross multiple nodes and storage devices. For example, in someembodiments, rather than the database engine applying changes to adatabase (or data pages thereof) and then sending the modified datapages to the storage layer, the application of changes to the storeddatabase (and data pages thereof) may be the responsibility of thestorage layer itself. In such embodiments, redo log records, rather thanmodified data pages, may be sent to the storage layer, after which redoprocessing (e.g., the application of the redo log records) may beperformed somewhat lazily and in a distributed manner (e.g., by abackground process). Log sequence numbers may be assigned to the redolog records from a log sequence number space. In some embodiments, crashrecovery (e.g., the rebuilding of data pages from stored redo logrecords) may also be performed by the storage layer and may also beperformed by a distributed (and, in some cases, lazy) backgroundprocess.

In some embodiments, because only redo logs (and not modified datapages) are sent to the storage layer, there may be much less networktraffic between the database tier and the storage layer than in existingdatabase systems. In some embodiments, each redo log may be on the orderof one-tenth the size of the corresponding data page for which itspecifies a change. Note that requests sent from the database tier andthe distributed storage system may be asynchronous and that multiplesuch requests may be in flight at a time.

In general, after being given a piece of data, a primary requirement ofa database is that it can eventually give that piece of data back. To dothis, the database may include several different components (or tiers),each of which performs a different function. For example, a traditionaldatabase may be thought of as having three tiers: a first tier forperforming query parsing, optimization and execution; a second tier forproviding transactionality, recovery, and durability; and a third tierthat provides storage, either on locally attached disks or onnetwork-attached storage. As noted above, previous attempts to scale atraditional database have typically involved replicating all three tiersof the database and distributing those replicated database instancesacross multiple machines.

In some embodiments, the systems described herein may partitionfunctionality of a database system differently than in a traditionaldatabase, and may distribute only a subset of the functional components(rather than a complete database instance) across multiple machines inorder to implement scaling. For example, in some embodiments, aclient-facing tier may be configured to receive a request specifyingwhat data is to be stored or retrieved, but not how to store or retrievethe data. This tier may perform request parsing and/or optimization(e.g., SQL parsing and optimization), while another tier may beresponsible for query execution. In some embodiments, a third tier maybe responsible for providing transactionality and consistency ofresults. For example, this tier may be configured to enforce some of theso-called ACID properties, in particular, the Atomicity of transactionsthat target the database, maintaining Consistency within the database,and ensuring Isolation between the transactions that target thedatabase. In some embodiments, a fourth tier may then be responsible forproviding Durability of the stored data in the presence of various sortsof faults. For example, this tier may be responsible for change logging,recovery from a database crash, managing access to the underlyingstorage volumes and/or space management in the underlying storagevolumes.

In various embodiments, a database instance may include multiplefunctional components (or layers), each of which provides a portion ofthe functionality of the database instance. In one such example, adatabase instance may include a query parsing and query optimizationlayer, a query execution layer, a transactionality and consistencymanagement layer, and a durability and space management layer. As notedabove, in some existing database systems, scaling a database instancemay involve duplicating the entire database instance one or more times(including all of the example layers), and then adding glue logic tostitch them together. In some embodiments, the systems described hereinmay instead offload the functionality of durability and space managementlayer from the database tier to a separate storage layer, and maydistribute that functionality across multiple storage nodes in thestorage layer.

In some embodiments, the database systems described herein may retainmuch of the structure of the upper half of the database instance, suchas query parsing and query optimization layer, a query execution layer,and a transactionality and consistency management layer, but mayredistribute responsibility for at least portions of the backup,restore, snapshot, recovery, and/or various space management operationsto the storage tier. Redistributing functionality in this manner andtightly coupling log processing between the database tier and thestorage tier may improve performance, increase availability and reducecosts, when compared to previous approaches to providing a scalabledatabase. For example, network and input/output bandwidth requirementsmay be reduced, since only redo log records (which are much smaller insize than the actual data pages) may be shipped across nodes orpersisted within the latency path of write operations. In addition, thegeneration of data pages can be done independently in the background oneach storage node (as foreground processing allows), without blockingincoming write operations. In some embodiments, the use oflog-structured, non-overwrite storage may allow backup, restore,snapshots, point-in-time recovery, and volume growth operations to beperformed more efficiently, e.g., by using metadata manipulation ratherthan movement or copying of a data page. In some embodiments, thestorage layer may also assume the responsibility for the replication ofdata stored on behalf of clients (and/or metadata associated with thatdata, such as redo log records) across multiple storage nodes. Forexample, data (and/or metadata) may be replicated locally (e.g., withina single “availability zone” in which a collection of storage nodesexecutes on its own physically distinct, independent infrastructure)and/or across availability zones in a single region or in differentregions.

In various embodiments, the database systems described herein maysupport a standard or custom application programming interface (API) fora variety of database operations. For example, the API may supportoperations for creating a database, creating a table, altering a table,creating a user, dropping a user, inserting one or more rows in a table,copying values, selecting data from within a table (e.g., querying atable), canceling or aborting a query, creating a snapshot, and/or otheroperations.

In some embodiments, the database tier of a database instance mayinclude a database engine head node server that receives read and/orwrite requests from various client programs (e.g., applications) and/orsubscribers (users), then parses them and develops an execution plan tocarry out the associated database operation(s). For example, thedatabase engine head node may develop the series of steps necessary toobtain results for complex queries and joins. In some embodiments, thedatabase engine head node may manage communications between the databasetier of the database system and clients/subscribers, as well ascommunications between the database tier and a separate distributedstorage system.

In some embodiments, the database engine head node may be responsiblefor receiving SQL requests from end clients through a JDBC or ODBCinterface and for performing SQL processing and transaction management(which may include locking) locally. However, rather than generatingdata pages locally, the database engine head node (or various componentsthereof) may generate redo log records and may ship them to theappropriate nodes of a separate distributed storage system. In someembodiments, a client-side driver for the distributed storage system maybe hosted on the database engine head node and may be responsible forrouting redo log records to the storage system node (or nodes) thatstore the segments (or data pages thereof) to which those redo logrecords are directed. For example, in some embodiments, each segment maybe mirrored (or otherwise made durable) on multiple storage system nodesthat form a protection group. In such embodiments, the client-sidedriver may keep track of the nodes on which each segment is stored andmay route redo logs to all of the nodes on which a segment is stored(e.g., asynchronously and in parallel, at substantially the same time),when a client request is received. As soon as the client-side driverreceives an acknowledgement back from a write quorum of the storagenodes in the protection group (which may indicate that the redo logrecord has been written to the storage node), it may send anacknowledgement of the requested change to the database tier (e.g., tothe database engine head node). For example, in embodiments in whichdata is made durable through the use of protection groups, the databaseengine head node may not be able to commit a transaction until andunless the client-side driver receives a reply from enough storage nodeinstances to constitute a write quorum, as may be defined in aprotection group policy for the data.

In some embodiments, the database tier (or more specifically, thedatabase engine head node) may include a cache in which recentlyaccessed data pages are held temporarily. In such embodiments, if awrite request is received that targets a data page held in such a cache,in addition to shipping a corresponding redo log record to the storagelayer, the database engine may apply the change to the copy of the datapage held in its cache. However, unlike in other database systems, adata page held in this cache may not ever be flushed to the storagelayer, and it may be discarded at any time (e.g., at any time after theredo log record for a write request that was most recently applied tothe cached copy has been sent to the storage layer and acknowledged).The cache may implement any of various locking mechanisms to controlaccess to the cache by at most one writer (or multiple readers) at atime, in different embodiments. Note, however, that in embodiments thatinclude such a cache, the cache may not be distributed across multiplenodes, but may exist only on the database engine head node for a givendatabase instance. Therefore, there may be no cache coherency orconsistency issues to manage.

In some embodiments, the database tier may support the use ofsynchronous or asynchronous read replicas in the system, e.g., read-onlycopies of data on different nodes of the database tier to which readrequests can be routed. In such embodiments, if the database engine headnode for a given database receives a read request directed to aparticular data page, it may route the request to any one (or aparticular one) of these read-only copies. In some embodiments, theclient-side driver in the database engine head node may be configured tonotify these other nodes about updates and/or invalidations to cacheddata pages (e.g., in order to prompt them to invalidate their caches,after which they may request updated copies of updated data pages fromthe storage layer).

In some embodiments, the client-side driver running on the databaseengine head node may expose a private interface to the storage tier. Insome embodiments, it may also expose a traditional iSCSI interface toone or more other components (e.g., other database engines or virtualcomputing services components). In some embodiments, storage for adatabase instance in the storage tier may be modeled as a single volumethat can grow in size without limits, and that can have an unlimitednumber of IOPS associated with it. When a volume is created, it may becreated with a specific size, with a specific availability/durabilitycharacteristic (e.g., specifying how it is replicated), and/or with anIOPS rate associated with it (e.g., both peak and sustained). Forexample, in some embodiments, a variety of different durability modelsmay be supported, and users/subscribers may be able to specify, fortheir database, a number of replication copies, zones, or regions and/orwhether replication is synchronous or asynchronous based upon theirdurability, performance and cost objectives.

In some embodiments, the client side driver may maintain metadata aboutthe volume and may directly send asynchronous requests to each of thestorage nodes necessary to fulfill read requests and write requestswithout requiring additional hops between storage nodes. The volumemetadata may indicate which protection groups, and their respectivestorage nodes, maintain which partitions of the volume. For example, insome embodiments, in response to a request to make a change to adatabase, the client-side driver may be configured to determine theprotection group, and its one or more nodes that are implementing thestorage for the targeted data page, and to route the redo log record(s)specifying that change to those storage nodes. The storage nodes maythen be responsible for applying the change specified in the redo logrecord to the targeted data page at some point in the future. As writesare acknowledged back to the client-side driver, the client-side drivermay advance the point at which the volume is durable and may acknowledgecommits back to the database tier. As previously noted, in someembodiments, the client-side driver may not ever send data pages to thestorage node servers. This may not only reduce network traffic, but mayalso remove the need for the checkpoint or background writer threadsthat constrain foreground-processing throughput in previous databasesystems.

In some embodiments, many read requests may be served by the databaseengine head node cache. However, write requests may require durability,since large-scale failure events may be too common to allow onlyin-memory replication. Therefore, the systems described herein may beconfigured to minimize the cost of the redo log record write operationsthat are in the foreground latency path by implementing data storage inthe storage tier as two regions: a small append-only log-structuredregion into which redo log records are written when they are receivedfrom the database tier, and a larger region in which log records arecoalesced together to create new versions of data pages in thebackground. In some embodiments, an in-memory structure may bemaintained for each data page that points to the last redo log recordfor that page, backward chaining log records until an instantiated datablock is referenced. This approach may provide good performance formixed read-write workloads, including in applications in which reads arelargely cached.

In some embodiments, because accesses to the log-structured data storagefor the redo log records may consist of a series of sequentialinput/output operations (rather than random input/output operations),the changes being made may be tightly packed together. It should also benoted that, in contrast to existing systems in which each change to adata page results in two input/output operations to persistent datastorage (one for the redo log and one for the modified data pageitself), in some embodiments, the systems described herein may avoidthis “write amplification” by coalescing data pages at the storage nodesof the distributed storage system based on receipt of the redo logrecords.

As previously noted, in some embodiments, the storage tier of thedatabase system may be responsible for taking database snapshots.However, because the storage tier implements log-structured storage,taking a snapshot of a data page (e.g., a data block) may includerecording a timestamp associated with the redo log record that was mostrecently applied to the data page/block (or a timestamp associated withthe most recent operation to coalesce multiple redo log records tocreate a new version of the data page/block), and preventing garbagecollection of the previous version of the page/block and any subsequentlog entries up to the recorded point in time. In such embodiments,taking a database snapshot may not require reading, copying, or writingthe data block, as would be required when employing an off-volume backupstrategy. In some embodiments, the space requirements for snapshots maybe minimal, since only modified data would require additional space,although user/subscribers may be able to choose how much additionalspace they want to keep for on-volume snapshots in addition to theactive data set. In different embodiments, snapshots may be discrete(e.g., each snapshot may provide access to all of the data in a datapage as of a specific point in time) or continuous (e.g., each snapshotmay provide access to all versions of the data that existing in a datapage between two points in time). In some embodiments, reverting to aprior snapshot may include recording a log record to indicate that allredo log records and data pages since that snapshot are invalid andgarbage collectable, and discarding all database cache entries after thesnapshot point. In such embodiments, no roll-forward may be requiredsince the storage system will, on a block-by-block basis, apply redo logrecords to data blocks as requested and in the background across allnodes, just as it does in normal forward read/write processing. Crashrecovery may thereby be made parallel and distributed across nodes.

One embodiment of a service system architecture that may be configuredto implement a network-based services-based database service isillustrated in FIG. 2. In the illustrated embodiment, a number ofclients (shown as clients 250 a-250 n) may be configured to interactwith a network-based services platform 200 via a network 260.Network-based services platform 200 may be configured to interface withone or more instances of a database service 210, a distributed storageservice 220 and/or one or more other virtual computing services 230.Distributed storage service may be implemented as log-structured storageusing a single log sequence number space. It is noted that where one ormore instances of a given component may exist, reference to thatcomponent herein may be made in either the singular or the plural.However, usage of either form is not intended to preclude the other.

In various embodiments, the components illustrated in FIG. 2 may beimplemented directly within computer hardware, as instructions directlyor indirectly executable by computer hardware (e.g., a microprocessor orcomputer system), or using a combination of these techniques. Forexample, the components of FIG. 2 may be implemented by a system thatincludes a number of computing nodes (or simply, nodes), each of whichmay be similar to the computer system embodiment illustrated in FIG. 12and described below. In various embodiments, the functionality of agiven service system component (e.g., a component of the databaseservice or a component of the storage service) may be implemented by aparticular node or may be distributed across several nodes. In someembodiments, a given node may implement the functionality of more thanone service system component (e.g., more than one database servicesystem component).

Generally speaking, clients 250 may encompass any type of clientconfigurable to submit network-based services requests to network-basedservices platform 200 via network 260, including requests for databaseservices (e.g., a request to generate a snapshot, etc.). For example, agiven client 250 may include a suitable version of a web browser, or mayinclude a plug-in module or other type of code module configured toexecute as an extension to or within an execution environment providedby a web browser. Alternatively, a client 250 (e.g., a database serviceclient) may encompass an application such as a database application (oruser interface thereof), a media application, an office application orany other application that may make use of persistent storage resourcesto store and/or access one or more databases. In some embodiments, suchan application may include sufficient protocol support (e.g., for asuitable version of Hypertext Transfer Protocol (HTTP)) for generatingand processing network-based services requests without necessarilyimplementing full browser support for all types of network-based data.That is, client 250 may be an application configured to interactdirectly with network-based services platform 200. In some embodiments,client 250 may be configured to generate network-based services requestsaccording to a Representational State Transfer (REST)-stylenetwork-based services architecture, a document- or message-basednetwork-based services architecture, or another suitable network-basedservices architecture.

In some embodiments, a client 250 (e.g., a database service client) maybe configured to provide access to network-based services-based storageof databases to other applications in a manner that is transparent tothose applications. For example, client 250 may be configured tointegrate with an operating system or file system to provide storage inaccordance with a suitable variant of the storage models describedherein. However, the operating system or file system may present adifferent storage interface to applications, such as a conventional filesystem hierarchy of files, directories and/or folders. In such anembodiment, applications may not need to be modified to make use of thestorage system service model. Instead, the details of interfacing tonetwork-based services platform 200 may be coordinated by client 250 andthe operating system or file system on behalf of applications executingwithin the operating system environment.

Clients 250 may convey network-based services requests (e.g., a snapshotrequest, parameters of a snapshot request, read request, restore asnapshot, etc.) to and receive responses from network-based servicesplatform 200 via network 260. In various embodiments, network 260 mayencompass any suitable combination of networking hardware and protocolsnecessary to establish network-based-based communications betweenclients 250 and platform 200. For example, network 260 may generallyencompass the various telecommunications networks and service providersthat collectively implement the Internet. Network 260 may also includeprivate networks such as local area networks (LANs) or wide areanetworks (WANs) as well as public or private wireless networks. Forexample, both a given client 250 and network-based services platform 200may be respectively provisioned within enterprises having their owninternal networks. In such an embodiment, network 260 may include thehardware (e.g., modems, routers, switches, load balancers, proxyservers, etc.) and software (e.g., protocol stacks, accounting software,firewall/security software, etc.) necessary to establish a networkinglink between given client 250 and the Internet as well as between theInternet and network-based services platform 200. It is noted that insome embodiments, clients 250 may communicate with network-basedservices platform 200 using a private network rather than the publicInternet. For example, clients 250 may be provisioned within the sameenterprise as a database service system (e.g., a system that implementsdatabase service 210 and/or distributed storage service 220). In such acase, clients 250 may communicate with platform 200 entirely through aprivate network 260 (e.g., a LAN or WAN that may use Internet-basedcommunication protocols but which is not publicly accessible).

Generally speaking, network-based services platform 200 may beconfigured to implement one or more service endpoints configured toreceive and process network-based services requests, such as requests toaccess data pages (or records thereof). For example, network-basedservices platform 200 may include hardware and/or software configured toimplement a particular endpoint, such that an HTTP-based network-basedservices request directed to that endpoint is properly received andprocessed. In one embodiment, network-based services platform 200 may beimplemented as a server system configured to receive network-basedservices requests from clients 250 and to forward them to components ofa system that implements database service 210, distributed storageservice 220 and/or another virtual computing service 230 for processing.In other embodiments, network-based services platform 200 may beconfigured as a number of distinct systems (e.g., in a cluster topology)implementing load balancing and other request management featuresconfigured to dynamically manage large-scale network-based servicesrequest processing loads. In various embodiments, network-based servicesplatform 200 may be configured to support REST-style or document-based(e.g., SOAP-based) types of network-based services requests.

In addition to functioning as an addressable endpoint for clients'network-based services requests, in some embodiments, network-basedservices platform 200 may implement various client management features.For example, platform 200 may coordinate the metering and accounting ofclient usage of network-based services, including storage resources,such as by tracking the identities of requesting clients 250, the numberand/or frequency of client requests, the size of data tables (or recordsthereof) stored or retrieved on behalf of clients 250, overall storagebandwidth used by clients 250, class of storage requested by clients250, or any other measurable client usage parameter. Platform 200 mayalso implement financial accounting and billing systems, or may maintaina database of usage data that may be queried and processed by externalsystems for reporting and billing of client usage activity. In certainembodiments, platform 200 may be configured to collect, monitor and/oraggregate a variety of storage service system operational metrics, suchas metrics reflecting the rates and types of requests received fromclients 250, bandwidth utilized by such requests, system processinglatency for such requests, system component utilization (e.g., networkbandwidth and/or storage utilization within the storage service system),rates and types of errors resulting from requests, characteristics ofstored and requested data pages or records thereof (e.g., size, datatype, etc.), or any other suitable metrics. In some embodiments suchmetrics may be used by system administrators to tune and maintain systemcomponents, while in other embodiments such metrics (or relevantportions of such metrics) may be exposed to clients 250 to enable suchclients to monitor their usage of database service 210, distributedstorage service 220 and/or another virtual computing service 230 (or theunderlying systems that implement those services).

In some embodiments, network-based services platform 200 may alsoimplement user authentication and access control procedures. Forexample, for a given network-based services request to access aparticular database, platform 200 may be configured to ascertain whetherthe client 250 associated with the request is authorized to access theparticular database. Platform 200 may determine such authorization by,for example, evaluating an identity, password or other credentialagainst credentials associated with the particular database, orevaluating the requested access to the particular database against anaccess control list for the particular database. For example, if aclient 250 does not have sufficient credentials to access the particulardatabase, platform 200 may reject the corresponding network-basedservices request, for example by returning a response to the requestingclient 250 indicating an error condition. Various access controlpolicies may be stored as records or lists of access control informationby database service 210, distributed storage service 220 and /or othervirtual computing services 230.

It is noted that while network-based services platform 200 may representthe primary interface through which clients 250 may access the featuresof a database system that implements database service 210, it need notrepresent the sole interface to such features. For example, an alternateAPI that may be distinct from a network-based services interface may beused to allow clients internal to the enterprise providing the databasesystem to bypass network-based services platform 200. Note that in manyof the examples described herein, distributed storage service 220 may beinternal to a computing system or an enterprise system that providesdatabase services to clients 250, and may not be exposed to externalclients (e.g., users or client applications). In such embodiments, theinternal “client” (e.g., database service 210) may access distributedstorage service 220 over a local or private network, shown as the solidline between distributed storage service 220 and database service 210(e.g., through an API directly between the systems that implement theseservices). In such embodiments, the use of distributed storage service220 in storing databases on behalf of clients 250 may be transparent tothose clients. In other embodiments, distributed storage service 220 maybe exposed to clients 250 through network-based services platform 200 toprovide storage of databases or other information for applications otherthan those that rely on database service 210 for database management.This is illustrated in FIG. 2 by the dashed line between network-basedservices platform 200 and distributed storage service 220. In suchembodiments, clients of the distributed storage service 220 may accessdistributed storage service 220 via network 260 (e.g., over theInternet). In some embodiments, a virtual computing service 230 may beconfigured to receive storage services from distributed storage service220 (e.g., through an API directly between the virtual computing service230 and distributed storage service 220) to store objects used inperforming computing services 230 on behalf of a client 250. This isillustrated in FIG. 2 by the dashed line between virtual computingservice 230 and distributed storage service 220. In some cases, theaccounting and/or credentialing services of platform 200 may beunnecessary for internal clients such as administrative clients orbetween service components within the same enterprise.

Although not illustrated, in various embodiments distributed storageservice 220 may be configured to interface with backup data store,system, service, or device. Various data, such as data pages, logrecords, and/or any other data maintained by distributed storage serviceinternal clients, such as database service 210 or other virtualcomputing services 230, and/or external clients such as clients 250 athrough 250 n, may be sent to a backup data store.

Note that in various embodiments, different storage policies may beimplemented by database service 210 and/or distributed storage service220. Examples of such storage policies may include a durability policy(e.g., a policy indicating the number of instances of a database (ordata page thereof) that will be stored and the number of different nodeson which they will be stored) and/or a load balancing policy (which maydistribute databases, or data pages thereof, across different nodes,volumes and/or disks in an attempt to equalize request traffic). Inaddition, different storage policies may be applied to different typesof stored items by various one of the services. For example, in someembodiments, distributed storage service 220 may implement a higherdurability for redo log records than for data pages.

FIG. 3 is a block diagram illustrating various components of a databasesystem that includes a database engine and a separate distributeddatabase storage service, according to one embodiment. In this example,database system 300 includes a respective database engine head node 320for each of several databases and a distributed storage service 310(which may or may not be visible to the clients of the database system,shown as database clients 350 a-350 n). As illustrated in this example,one or more of database clients 350 a-350 n may access a database headnode 320 (e.g., head node 320 a, head node 320 b, or head node 320 c,each of which is a component of a respective database instance) vianetwork 360 (e.g., these components may be network-addressable andaccessible to the database clients 350 a-350 n). However, distributedstorage service 310, which may be employed by the database system tostore a database volume (such as data pages of one or more databases, aswell as redo log records and/or other metadata associated therewith) onbehalf of database clients 350 a-350 n, and to perform other functionsof the database system as described herein, may or may not benetwork-addressable and accessible to the storage clients 350 a-350 n,in different embodiments. For example, in some embodiments, distributedstorage service 310 may perform various storage, access, change logging,recovery, log record manipulation, and/or space management operations ina manner that is invisible to storage clients 350 a-350 n.

As previously noted, each database instance may include a singledatabase engine head node 320 that receives requests (e.g., a snapshotrequest, etc.) from various client programs (e.g., applications) and/orsubscribers (users), then parses them, optimizes them, and develops anexecution plan to carry out the associated database operation(s). In theexample illustrated in FIG. 3, a query parsing, optimization, andexecution component 305 of database engine head node 320 a may performthese functions for queries that are received from database client 350 aand that target the database instance of which database engine head node320 a is a component. In some embodiments, query parsing, optimization,and execution component 305 may return query responses to databaseclient 350 a, which may include write acknowledgements, requested datapages (or portions thereof), error messages, and or other responses, asappropriate. As illustrated in this example, database engine head node320 a may also include a client-side storage service driver 325, whichmay route read requests and/or redo log records to various storage nodeswithin distributed storage service 310, receive write acknowledgementsfrom distributed storage service 310, receive requested data pages fromdistributed storage service 310, and/or return data pages, errormessages, or other responses to query parsing, optimization, andexecution component 305 (which may, in turn, return them to databaseclient 350 a). Client-side storage device may maintain mappinginformation about the database volume stored in distributed storageservice 310, such that a particular protection group maintaining apartition of the database volume may be determined. Read requests andredo log records may then be routed to storage nodes that are members ofthe protection group according to the partition of user data to whichthe read request is directed or to which the redo log record pertains.

In this example, database engine head node 320 a includes a data pagecache 335, in which data pages that were recently accessed may betemporarily held. As illustrated in FIG. 3, database engine head node320 a may also include a transaction and consistency managementcomponent 330, which may be responsible for providing transactionalityand consistency in the database instance of which database engine headnode 320 a is a component. For example, this component may beresponsible for ensuring the Atomicity, Consistency, and Isolationproperties of the database instance and the transactions that aredirected that the database instance. As illustrated in FIG. 3, databaseengine head node 320 a may also include a transaction log 340 and anundo log 345, which may be employed by transaction and consistencymanagement component 330 to track the status of various transactions androll back any locally cached results of transactions that do not commit.

Note that each of the other database engine head nodes 320 illustratedin FIG. 3 (e.g., 320 b and 320 c) may include similar components and mayperform similar functions for queries received by one or more ofdatabase clients 350 a-350 n and directed to the respective databaseinstances of which it is a component.

In some embodiments, the distributed storage systems described hereinmay organize data in various logical volumes, extents (which may includepartitions of the user data space in the volume and a segmentation ofthe log for the volume) made durable among a protection group of storagenodes, segments (which may be data stored on an individual storage nodeof a protection group) and pages for storage on one or more storagenodes. For example, in some embodiments, each database is represented bya logical volume, and each logical volume is partitioned over acollection of storage nodes into extents. A protection group may becomposed of different storage nodes in the distributed storage servicethat together make an extent durable. Multiple segments, each of whichlives on a particular one of the storage nodes in a protection group,are used to make the extent durable.

In some embodiments, each data page is stored in a segment, such thateach segment stores a collection of one or more data pages and a changelog (also referred to as a redo log) (e.g., a log of redo log records)for each data page that it stores. Thus, change logs may be log recordssegmented to the protection group of which the segment is a member. Asdescribed in detail herein, the storage nodes may be configured toreceive redo log records (which may also be referred to herein as ULRs)and to coalesce them to create new versions of the corresponding datapages and/or additional or replacement log records (e.g., lazily and/orin response to a request for a data page or a database crash). In someembodiments, data pages and/or change logs may be mirrored acrossmultiple storage nodes, according to a variable configuration, such asin a protection group (which may be specified by the client on whosebehalf the databases are being maintained in the database system). Forexample, in different embodiments, one, two, or three copies of the dataor change logs may be stored in each of one, two, or three differentavailability zones or regions, according to a default configuration, anapplication-specific durability preference, or a client-specifieddurability preference.

As used herein, the following terms may be used to describe theorganization of data by a distributed storage system, according tovarious embodiments.

Volume: A volume is a logical concept representing a highly durable unitof storage that a user/client/application of the storage systemunderstands. More specifically, a volume is a distributed store thatappears to the user/client/application as a single consistent orderedlog of write operations to various user pages of a database. Each writeoperation may be encoded in a User Log Record (ULR), which represents alogical, ordered mutation to the contents of a single user page withinthe volume. As noted above, a ULR may also be referred to herein as aredo log record. Each ULR may include a unique identifier (e.g., aLogical Sequence Number (LSN)) assigned from a log sequence numberspace. Each ULR may be persisted to one or more synchronous segments inthe log-structured distributed store that form a Protection Group (PG)maintaining the partition of user data space (i.e. extent) to which theupdate indicate by the log record pertains in order to provide highdurability and availability for the ULR. A volume may provide anLSN-type read/write interface for a variable-size contiguous range ofbytes.

In some embodiments, a volume may consist of multiple extents, each madedurable through a protection group. In such embodiments, a volume mayrepresent a unit of storage composed of a mutable contiguous sequence ofVolume Extents. Reads and writes that are directed to a volume may bemapped into corresponding reads and writes to the constituent volumeextents. In some embodiments, the size of a volume may be changed byadding or removing volume extents from the end of the volume.

Segment: A segment is a limited-durability unit of storage assigned to asingle storage node. Multiple segments may be implemented in aprotection group to persist an extent. More specifically, a segmentprovides limited best-effort durability (e.g., a persistent, butnon-redundant single point of failure that is a storage node) for aspecific fixed-size byte range of data. This data may in some cases be amirror of user-addressable data, or it may be other data, such as volumemetadata or erasure coded bits, in various embodiments. A given segmentmay live on exactly one storage node. Within a storage node, multiplesegments may live on each SSD, and each segment may be restricted to oneSSD (e.g., a segment may not span across multiple SSDs). In someembodiments, a segment may not be required to occupy a contiguous regionon an SSD; rather there may be an allocation map in each SSD describingthe areas that are owned by each of the segments. As noted above, aprotection group may consist of multiple segments spread across multiplestorage nodes. In some embodiments, a segment may provide an LSN-typeread/write interface for a fixed-size contiguous range of bytes (wherethe size is defined at creation). In some embodiments, each segment maybe identified by a Segment UUID (e.g., a universally unique identifierof the segment).

Storage page: A storage page is a block of memory, generally of fixedsize. In some embodiments, each page is a block of memory (e.g., ofvirtual memory, disk, or other physical memory) of a size defined by theoperating system, and may also be referred to herein by the term “datablock”. More specifically, a storage page may be a set of contiguoussectors. It may serve as the unit of allocation in SSDs, as well as theunit in log pages for which there is a header and metadata. In someembodiments, and in the context of the database systems describedherein, the term “page” or “storage page” may refer to a similar blockof a size defined by the database configuration, which may typically amultiple of 2, such as 4096, 8192, 16384, or 32768 bytes.

Log page: A log page is a type of storage page that is used to store logrecords (e.g., redo log records or undo log records). In someembodiments, log pages may be identical in size to storage pages. Eachlog page may include a header containing metadata about that log page,e.g., metadata identifying the segment to which it belongs. Note that alog page is a unit of organization and may not necessarily be the unitof data included in write operations. For example, in some embodiments,during normal forward processing, write operations may write to the tailof the log one sector at a time.

Log Records: Log records (e.g., the individual elements of a log page)may be of several different classes. For example, User Log Records(ULRs), which are created and understood by users/clients/applicationsof the storage system, may be used to indicate changes to user data in avolume. Log records may include metadata, such as pointers or backlinks, that indicate a previous LSN for log record maintained at aparticular segment and/or the previous LSN in the log sequence numberspace. Control Log Records (CLRs), which are generated by the storagesystem, may also contain control information used to keep track ofmetadata such as the current unconditional volume durable LSN (VDL).Null Log Records (NLRB) may in some embodiments be used as padding tofill in unused space in a log sector or log page. In some embodiments,there may be various types of log records within each of these classes,and the type of a log record may correspond to a function that needs tobe invoked to interpret the log record. For example, one type mayrepresent all the data of a user page in compressed format using aspecific compression format; a second type may represent new values fora byte range within a user page; a third type may represent an incrementoperation to a sequence of bytes interpreted as an integer; and a fourthtype may represent copying one byte range to another location within thepage. In some embodiments, log record types may be identified by GUIDs(rather than by integers or enums), which may simplify versioning anddevelopment, especially for ULRs.

Payload: The payload of a log record is the data or parameter valuesthat are specific to the log record or to log records of a particulartype. For example, in some embodiments, there may be a set of parametersor attributes that most (or all) log records include, and that thestorage system itself understands. These attributes may be part of acommon log record header/structure, which may be relatively smallcompared to the sector size. In addition, most log records may includeadditional parameters or data specific to that log record type, and thisadditional information may be considered the payload of that log record.In some embodiments, if the payload for a particular ULR is larger thanthe user page size, it may be replaced by an absolute ULR (an AULR)whose payload includes all the data for the user page. This may enablethe storage system to enforce an upper limit on the size of the payloadfor ULRs that is equal to the size of user pages.

Note that when storing log records in the segment log, the payload maybe stored along with the log header, in some embodiments. In otherembodiments, the payload may be stored in a separate location, andpointers to the location at which that payload is stored may be storedwith the log header. In still other embodiments, a portion of thepayload may be stored in the header, and the remainder of the payloadmay be stored in a separate location. If the entire payload is storedwith the log header, this may be referred to as in-band storage;otherwise the storage may be referred to as being out-of-band. In someembodiments, the payloads of most large AULRs may be stored out-of-bandin the cold zone of log (which is described below).

User pages: User pages are the byte ranges (of a fixed size) andalignments thereof for a particular volume that are visible tousers/clients of the storage system. User pages are a logical concept,and the bytes in particular user pages may or not be stored in anystorage page as-is. The size of the user pages for a particular volumemay be independent of the storage page size for that volume. In someembodiments, the user page size may be configurable per volume, anddifferent segments on a storage node may have different user page sizes.In some embodiments, user page sizes may be constrained to be a multipleof the sector size (e.g., 4 KB), and may have an upper limit (e.g., 64KB). The storage page size, on the other hand, may be fixed for anentire storage node and may not change unless there is a change to theunderlying hardware.

Data page: A data page is a type of storage page that is used to storeuser page data in compressed form. In some embodiments every piece ofdata stored in a data page is associated with a log record, and each logrecord may include a pointer to a sector within a data page (alsoreferred to as a data sector). In some embodiments, data pages may notinclude any embedded metadata other than that provided by each sector.There may be no relationship between the sectors in a data page.Instead, the organization into pages may exist only as an expression ofthe granularity of the allocation of data to a segment.

Storage node: A storage node is a single virtual machine that on whichstorage node server code is deployed. Each storage node may containmultiple locally attached SSDs, and may provide a network API for accessto one or more segments. In some embodiments, various nodes may be on anactive list or on a degraded list (e.g., if they are slow to respond orare otherwise impaired, but are not completely unusable). In someembodiments, the client-side driver may assist in (or be responsiblefor) classifying nodes as active or degraded, for determining if andwhen they should be replaced, and/or for determining when and how toredistribute data among various nodes, based on observed performance.Multiple storage nodes may together implement a protection group, insome embodiments.

SSD: As referred to herein, the term “SSD” may refer to a local blockstorage volume as seen by the storage node, regardless of the type ofstorage employed by that storage volume, e.g., disk, a solid-statedrive, a battery-backed RAM, a non-volatile RAM device (e.g., one ormore NV-DIMMs) or another type of persistent storage device. An SSD isnot necessarily mapped directly to hardware. For example, a singlesolid-state storage device might be broken up into multiple localvolumes where each volume is split into and striped across multiplesegments, and/or a single drive may be broken up into multiple volumessimply for ease of management, in different embodiments. In someembodiments, each SSD may store an allocation map at a single fixedlocation. This map may indicate which storage pages that are owned byparticular segments, and which of these pages are log pages (as opposedto data pages). In some embodiments, storage pages may be pre-allocatedto each segment so that forward processing may not need to wait forallocation. Any changes to the allocation map may need to be madedurable before newly allocated storage pages are used by the segments.

One embodiment of a distributed storage system is illustrated by theblock diagram in FIG. 4. In at least some embodiments, storage nodes430-450 may store data for different clients as part of a multi-tenantstorage service. For example, the various segments discussed above andbelow with regard to FIG. 7, may correspond to different protectiongroups and volumes for different clients. As noted above, some storagenodes may perform garbage collection independent from other storagenodes. Consider the scenario where a storage node maintains data for twodifferent clients. One client's data may be actively accessed/modified,causing the log structure for that data to grow quickly. Though, theother data maintained for the other client may be accessed infrequently,garbage collection may be performed to reclaim log pages associated withthe other data in order to make more data pages available for the moreactive log.

In some embodiments, a database system 400 may be a client ofdistributed storage system 410, which communicates with a databaseengine head node 420 over interconnect 460. As in the exampleillustrated in FIG. 3, database engine head node 420 may include aclient-side storage service driver 425. In this example, distributedstorage system 410 includes multiple storage system server nodes(including those shown as 430, 440, and 450), each of which includesstorage for data pages and redo logs for the segment(s) it stores, andhardware and/or software configured to perform various segmentmanagement functions. For example, each storage system server node mayinclude hardware and/or software configured to perform at least aportion of any or all of the following operations: replication (locally,e.g., within the storage node), coalescing of redo logs to generate datapages, snapshots (e.g., creating, restoration, deletion, etc.), logmanagement (e.g., manipulating log records), crash recovery (e.g.,determining candidate log records for volume recovery), and/or spacemanagement (e.g., for a segment). Each storage system server node mayalso have multiple attached storage devices (e.g., SSDs) on which datablocks may be stored on behalf of clients (e.g., users, clientapplications, and/or database service subscribers).

In the example illustrated in FIG. 4, storage system server node 430includes data page(s) 433, segment redo log(s) 435, segment managementfunctions 437, and attached SSDs 471-478. Again note that the label“SSD” may or may not refer to a solid-state drive, but may moregenerally refer to a local block storage volume, regardless of itsunderlying hardware. Similarly, storage system server node 440 includesdata page(s) 443, segment redo log(s) 445, segment management functions447, and attached SSDs 481-488; and storage system server node 450includes data page(s) 453, segment redo log(s) 455, segment managementfunctions 457, and attached SSDs 491-498.

As previously noted, in some embodiments, a sector is the unit ofalignment on an SSD and may be the maximum size on an SSD that can bewritten without the risk that the write will only be partiallycompleted. For example, the sector size for various solid-state drivesand spinning media may be 4 KB. In some embodiments of the distributedstorage systems described herein, each and every sector may include havea 64-bit (8 byte) CRC at the beginning of the sector, regardless of thehigher-level entity of which the sector is a part. In such embodiments,this CRC (which may be validated every time a sector is read from SSD)may be used in detecting corruptions. In some embodiments, each andevery sector may also include a “sector type” byte whose valueidentifies the sector as a log sector, a data sector, or anuninitialized sector. For example, in some embodiments, a sector typebyte value of 0 may indicate that the sector is uninitialized.

In some embodiments, each of the storage system server nodes in thedistributed storage system may implement a set of processes running onthe node server's operating system that manage communication with thedatabase engine head node, e.g., to receive redo logs, send back datapages, etc. In some embodiments, all data blocks written to thedistributed storage system may be backed up to long-term and/or archivalstorage (e.g., in a remote key-value durable backup storage system).

Distributed storage system 410 may also implement a storage controlplane. Storage control plane may be one or more compute nodes configuredto perform a variety of different storage system management functions.For example, storage control plane may implement a volume manager, whichmay be configured to maintain mapping information for a volume as it ispersisted in varying different, extents, segments, and protectiongroups. A volume manager may be configured to communicate with a clientof storage system 410, such as client-side driver 425 in order to“mount” the volume for the client, providing client-side driver 425 withmapping information, protection group policies, and various otherinformation necessary to send write and read requests to storage nodes430-450. Storage control plane may also implement a recovery module orservice for storage system clients, such as database system 400. Forexample, the various methods and techniques described below with regardto FIGS. 9-11 may be performed, request, or coordinated by the recoverymodule.

Client-side storage service driver 425 may also implement variousrecovery techniques for distributed storage system 410, such as thosediscussed below in FIGS. 9-11. For example, upon recovery from a failureof database engine head node 420, CSD 425 may determine and provide arecovery point from which database engine head node 420 may access aconsistent view of the database volume stored in distributed storagesystem 410. In order to determine the recovery point for the log, CSD420 may request candidate redo log records from each protection groupmaintaining an extent of a database volume. For instance, one or morestorage nodes of each protection group may be sent the request. Storagesystem server node 430, for example, may receive the request forcandidate log records for log recovery of the log for the databasevolume.

Using metadata maintained in an in-memory data structure, or by readingmetadata in the log records stored at the storage node 430 thatindicates previous log sequence numbers of log records also maintainedat the protection group, different log records may be selected accordingto an identified position of the log records in a recovery sequence forthe protection group. For instance, the recovery sequence may bedetermined from the metadata of previous LSNs, and indicate a chain orordering of log records that are maintained at the protection group. Bytraversing the chain to find the first break or gap in the log records,the log record in the chain prior to the gap may be a completion pointfor the protection group. Log records above the completion (and the gap)may not be durably persisted among the segments in the protection group(e.g., may not satisfy a write quorum for the PG) and may not beeligible for selection as a candidate log record for log recovery. LSNsbelow the completion point may be eligible. Eligible log records mayfiltered down to a smaller subset of log records. In some embodiments, astorage node, such as storage node 430 may filter or remove log recordsthat do not indicate completion of a system transaction (i.e., they arelog records indicating a change in the middle of a system transaction)from selected candidate. Metadata about a the protection group or avolume, such as the current unconditional volume durable LSN (VDL), maybe used to filter the eligible log records. For example, in someembodiments, all log records with LSNs below the current unconditionalvolume durable LSN (VDL) may be excluded. The candidate log records forlog recovery may be sent back to client-side driver 425.

Client-side driver 425 may then evaluate the candidate log recordsreceived from the protection groups to identify the recovery point inthe log for the database volume according to the log sequence numberfrom LSNs are assigned. For example, starting from the candidate logrecord with the lowest LSN, the log records may be traversed untilreaching the first break in the sequence. The log record before thefirst break, may be identified as the recovery point for the log. FIGS.9-11 discuss in greater detail the various methods and techniques that aclient-side driver or other system module or device may implementrecovery for a log-structured distributed storage system using a singlelog sequence number space, and as such the previous discussion is notintended to be limiting.

FIG. 5 is a block diagram illustrating the use of a separate distributedstorage system in a database system, according to one embodiment. Inthis example, one or more client processes 510 may store data to one ormore databases maintained by a database system that includes a databaseengine 520 and a distributed storage system 530. In the exampleillustrated in FIG. 5, database engine 520 includes database tiercomponents 560 and client-side driver 540 (which serves as the interfacebetween distributed storage system 530 and database tier components560). In some embodiments, database tier components 560 may performfunctions such as those performed by query parsing, optimization andexecution component 305 and transaction and consistency managementcomponent 330 of FIG. 3, and/or may store data pages, transaction logsand/or undo logs (such as those stored by data page cache 335,transaction log 340 and undo log 345 of FIG. 3).

In this example, one or more client processes 510 may send databasequery requests 515 (which may include read and/or write requeststargeting data stored on one or more of the storage nodes 535 a-535 n)to database tier components 560, and may receive database queryresponses 517 from database tier components 560 (e.g., responses thatinclude write acknowledgements and/or requested data). Each databasequery request 515 that includes a request to write to a data page may beparsed and optimized to generate one or more write record requests 541,which may be sent to client-side driver 540 for subsequent routing todistributed storage system 530. In this example, client-side driver 540may generate one or more redo log records 531 corresponding to eachwrite record request 541, and may send them to specific ones of thestorage nodes 535 of specific protection groups storing the partitionuser data of user data space to which the write record request pertainsin distributed storage system 530. Client-side driver 540 may generatemetadata for each of the redo log records that includes an indication ofa previous log sequence number of a log record maintained at thespecific protection group. Distributed storage system 530 may return acorresponding write acknowledgement(s) 523 for each redo log record 531to database engine 520 (specifically to client-side driver 540).Client-side driver 540 may pass these write acknowledgements to databasetier components 560 (as write responses 542), which may then sendcorresponding responses (e.g., write acknowledgements) to one or moreclient processes 510 as one of database query responses 517.

In this example, each database query request 515 that includes a requestto read a data page may be parsed and optimized to generate one or moreread record requests 543, which may be sent to client-side driver 540for subsequent routing to distributed storage system 530. In thisexample, client-side driver 540 may send these requests to specific onesof the storage nodes 535 of distributed storage system 530, anddistributed storage system 530 may return the requested data pages 533to database engine 520 (specifically to client-side driver 540).Client-side driver 540 may send the returned data pages to the databasetier components 560 as return data records 544, and database tiercomponents 560 may then send the data pages to one or more clientprocesses 510 as database query responses 517.

In some embodiments, various error and/or data loss messages 534 may besent from distributed storage system 530 to database engine 520(specifically to client-side driver 540). These messages may be passedfrom client-side driver 540 to database tier components 560 as errorand/or loss reporting messages 545, and then to one or more clientprocesses 510 along with (or instead of) a database query response 517.

In some embodiments, the APIs 531-534 of distributed storage system 530and the APIs 541-545 of client-side driver 540 may expose thefunctionality of the distributed storage system 530 to database engine520 as if database engine 520 were a client of distributed storagesystem 530. For example, database engine 520 (through client-side driver540) may write redo log records or request data pages through these APIsto perform (or facilitate the performance of) various operations of thedatabase system implemented by the combination of database engine 520and distributed storage system 530 (e.g., storage, access, changelogging, recovery, and/or space management operations). As illustratedin FIG. 5, distributed storage system 530 may store data blocks onstorage nodes 535 a-535 n, each of which may have multiple attachedSSDs. In some embodiments, distributed storage system 530 may providehigh durability for stored data block through the application of varioustypes of redundancy schemes.

Note that in various embodiments, the API calls and responses betweendatabase engine 520 and distributed storage system 530 (e.g., APIs531-534) and/or the API calls and responses between client-side driver540 and database tier components 560 (e.g., APIs 541-545) in FIG. 5 maybe performed over a secure proxy connection (e.g., one managed by agateway control plane), or may be performed over the public network or,alternatively, over a private channel such as a virtual private network(VPN) connection. These and other APIs to and/or between components ofthe database systems described herein may be implemented according todifferent technologies, including, but not limited to, Simple ObjectAccess Protocol (SOAP) technology and Representational state transfer(REST) technology. For example, these APIs may be, but are notnecessarily, implemented as SOAP APIs or RESTful APIs. SOAP is aprotocol for exchanging information in the context of network-basedservices. REST is an architectural style for distributed hypermediasystems. A RESTful API (which may also be referred to as a RESTfulnetwork-based service) is a network-based service API implemented usingHTTP and REST technology. The APIs described herein may in someembodiments be wrapped with client libraries in various languages,including, but not limited to, C, C++, Java, C# and Perl to supportintegration with database engine 520 and/or distributed storage system530.

As noted above, in some embodiments, the functional components of adatabase system may be partitioned between those that are performed bythe database engine and those that are performed in a separate,distributed storage system. In one specific example, in response toreceiving a request from a client process (or a thread thereof) toinsert something into a database (e.g., to update a single data block byadding a record to that data block), one or more components of thedatabase engine head node may perform query parsing, optimization, andexecution, and may send each portion of the query to a transaction andconsistency management component. The transaction and consistencymanagement component may ensure that no other client process (or threadthereof) is trying to modify the same row at the same time. For example,the transaction and consistency management component may be responsiblefor ensuring that this change is performed atomically, consistently,durably, and in an isolated manner in the database. For example, thetransaction and consistency management component may work together withthe client-side storage service driver of the database engine head nodeto generate a redo log record to be sent to one of the nodes in thedistributed storage service and to send it to the distributed storageservice (along with other redo logs generated in response to otherclient requests) in an order and/or with timing that ensures the ACIDproperties are met for this transaction. Upon receiving the redo logrecord (which may be considered an “update record” by the storageservice), the corresponding storage node may update the data block, andmay update a redo log for the data block (e.g., a record of all changesdirected to the data block). In some embodiments, the database enginemay be responsible for generating an undo log record for this change,and may also be responsible for generating a redo log record for theundo log both of which may be used locally (in the database tier) forensuring transactionality. However, unlike in traditional databasesystems, the systems described herein may shift the responsibility forapplying changes to data blocks to the storage system (rather thanapplying them at the database tier and shipping the modified data blocksto the storage system).

A variety of different allocation models may be implemented for an SSD,in different embodiments. For example, in some embodiments, log entrypages and physical application pages may be allocated from a single heapof pages associated with an SSD device. This approach may have theadvantage of leaving the relative amount of storage consumed by logpages and data pages to remain unspecified and to adapt automatically tousage. It may also have the advantage of allowing pages to remainunprepared until they are used, and repurposed at will withoutpreparation. In other embodiments, an allocation model may partition thestorage device into separate spaces for log entries and data pages. Oncesuch allocation model is illustrated by the block diagram in FIG. 6 anddescribed below.

FIG. 6 is a block diagram illustrating how data and metadata may bestored on a given storage node (or persistent storage device) of adistributed storage system, according to one embodiment. In thisexample, SSD storage space 600 stores an SSD header and other fixedmetadata in the portion of the space labeled 610. It stores log pages inthe portion of the space labeled 620, and includes a space labeled 630that is initialized and reserved for additional log pages. One portionof SSD storage space 600 (shown as 640) is initialized, but unassigned,and another portion of the space (shown as 650) is uninitialized andunassigned. Finally, the portion of SSD storage space 600 labeled 660stores data pages.

In this example, the first usable log page slot is noted as 615, and thelast used log page slot (ephemeral) is noted as 625. The last reservedlog page slot is noted as 635, and the last usable log page slot isnoted as 645. In this example, the first used data page slot (ephemeral)is noted as 665. In some embodiments, the positions of each of theseelements (615, 625, 635, 645, and 665) within SSD storage space 600 maybe identified by a respective pointer.

In allocation approach illustrated in FIG. 6, valid log pages may bepacked into the beginning of the flat storage space. Holes that open updue to log pages being freed may be reused before additional log pageslots farther into the address space are used. For example, in the worstcase, the first n log page slots contain valid log data, where n is thelargest number of valid log pages that have ever simultaneously existed.In this example, valid data pages may be packed into the end of the flatstorage space. Holes that open up due to data pages being freed may bereused before additional data page slots lower in the address space areused. For example, in the worst case, the last m data pages containvalid data, where m is the largest number of valid data pages that haveever simultaneously existed.

In some embodiments, before a log page slot can become part of thepotential set of valid log page entries, it may need to be initializedto a value that cannot be confused for a valid future log entry page.This is implicitly true for recycled log page slots, since a retired logpage has enough metadata to never be confused for a new valid log page.However, when a storage device is first initialized, or when space isreclaimed that had potentially been used to store application datapages, the log page slots may need to be initialized before they areadded to the log page slot pool. In some embodiments,rebalancing/reclaiming log space may be performed as a background task.

In the example illustrated in FIG. 6, the current log page slot poolincludes the area between the first usable log page slot (at 615) andthe last reserved log page slot (625). In some embodiments, this poolmay safely grow up to last usable log page slot (625) withoutre-initialization of new log page slots (e.g., by persisting an updateto the pointer that identifies the last reserved log page slot, 635). Inthis example, beyond the last usable log page slot (which is identifiedby pointer 645), the pool may grow up to the first used data page slot(which is identified by pointer 665) by persisting initialized log pageslots and persistently updating the pointer for the last usable log pageslot (645). In this example, the previously uninitialized and unassignedportion of the SSD storage space 600 shown as 650 may be pressed intoservice to store log pages. In some embodiments, the current log pageslot pool may be shrunk down to the position of the last used log pageslot (which is identified by pointer) by persisting an update to thepointer for the last reserved log page slot (635).

In the example illustrated in FIG. 6, the current data page slot poolincludes the area between the last usable log page slot (which isidentified by pointer 645) and the end of SSD storage space 600. In someembodiments, the data page pool may be safely grown to the positionidentified by the pointer to the last reserved log page slot (635) bypersisting an update to the pointer to the last usable log page slot(645). In this example, the previously initialized, but unassignedportion of the SSD storage space 600 shown as 640 may be pressed intoservice to store data pages. Beyond this, the pool may be safely grownto the position identified by the pointer to the last used log page slot(625) by persisting updates to the pointers for the last reserved logpage slot (635) and the last usable log page slot (645), effectivelyreassigning the portions of SSD storage space 600 shown as 630 and 640to store data pages, rather than log pages. In some embodiments, thedata page slot pool may be safely shrunk down to the position identifiedby the pointer to the first used data page slot (665) by initializingadditional log page slots and persisting an update to the pointer to thelast usable log page slot (645).

In embodiments that employ the allocation approach illustrated in FIG.6, page sizes for the log page pool and the data page pool may beselected independently, while still facilitating good packing behavior.In such embodiments, there may be no possibility of a valid log pagelinking to a spoofed log page formed by application data, and it may bepossible to distinguish between a corrupted log and a valid log tailthat links to an as-yet-unwritten next page. In embodiments that employthe allocation approach illustrated in FIG. 6, at startup, all of thelog page slots up to the position identified by the pointer to the lastreserved log page slot (635) may be rapidly and sequentially read, andthe entire log index may be reconstructed (including inferredlinking/ordering). In such embodiments, there may be no need forexplicit linking between log pages, since everything can be inferredfrom LSN sequencing constraints.

In some embodiments, a segment may consist of three main parts (orzones): one that contains a hot log, one that contains a cold log, andone that contains user page data. Zones are not necessarily contiguousregions of an SSD. Rather, they can be interspersed at the granularityof the storage page. In addition, there may be a root page for eachsegment that stores metadata about the segment and its properties. Forexample, the root page for a segment may store the user page size forthe segment, the number of user pages in the segment, the currentbeginning/head of the hot log zone (which may be recorded in the form ofa flush number), the volume epoch, and/or access control metadata.

In some embodiments, the hot log zone may accept new writes from theclient as they are received by the storage node. Both Delta User LogRecords (DULRs), which specify a change to a user/data page in the formof a delta from the previous version of the page, and Absolute User LogRecords (AULRs), which specify the contents of a complete user/datapage, may be written completely into the log. Log records may be addedto this zone in approximately the order they are received (e.g., theyare not sorted by LSN) and they can span across log pages. The logrecords may be self-describing, e.g., they may contain an indication oftheir own size. In some embodiments, no garbage collection is performedin this zone. Instead, space may be reclaimed by truncating from thebeginning of the log after all required log records have been copiedinto the cold log. Log sectors in the hot zone may be annotated with themost recent known unconditional VDL each time a sector is written.Conditional VDL CLRs may be written into the hot zone as they arereceived, but only the most recently written VDL CLR may be meaningful.

In some embodiments, every time a new log page is written, it may beassigned a flush number. The flush number may be written as part ofevery sector within each log page. Flush numbers may be used todetermine which log page was written later when comparing two log pages.Flush numbers are monotonically increasing and scoped to an SSD (orstorage node). For example, a set of monotonically increasing flushnumbers is shared between all segments on an SSD (or all segments on astorage node).

In some embodiments, in the cold log zone, log records may be stored inincreasing order of their LSNs. In this zone, AULRs may not necessarilystore data in-line, depending on their size. For example, if they havelarge payloads, all or a portion of the payloads may be stored in thedata zone and they may point to where their data is stored in the datazone. In some embodiments, log pages in the cold log zone may be writtenone full page at a time, rather than sector-by-sector. Because log pagesin the cold zone are written a full page at a time, any log page in thecold zone for which the flush numbers in all sectors are not identicalmay be considered to be an incompletely written page and may be ignored.In some embodiments, in the cold log zone, DULRs may be able to spanacross log pages (up to a maximum of two log pages). However, AULRs maynot be able to span log sectors, e.g., so that a coalesce operation willbe able to replace a DULR with an AULR in a single atomic write.

In some embodiments, the cold log zone is populated by copying logrecords from the hot log zone. In such embodiments, only log recordswhose LSN is less than or equal to the current unconditional volumedurable LSN (VDL) may be eligible to be copied to the cold log zone.When moving log records from the hot log zone to the cold log zone, somelog records (such as many CLRs) may not need to be copied because theyare no longer necessary. In addition, some additional coalescing of userpages may be performed at this point, which may reduce the amount ofcopying required. In some embodiments, once a given hot zone log pagehas been completely written and is no longer the newest hot zone logpage, and all ULRs on the hot zone log page have been successfullycopied to the cold log zone, the hot zone log page may be freed andreused.

In some embodiments, garbage collection may be done in the cold log zoneto reclaim space occupied by obsolete log records, e.g., log recordsthat no longer need to be stored in the SSDs of the storage tier. Forexample, a log record may become obsolete when there is a subsequentAULR for the same user page and the version of the user page representedby the log record is not needed for retention on SSD. In someembodiments, a garbage collection process may reclaim space by mergingtwo or more adjacent log pages and replacing them with fewer new logpages containing all of the non-obsolete log records from the log pagesthat they are replacing. The new log pages may be assigned new flushnumbers that are larger than the flush numbers of the log pages they arereplacing. After the write of these new log pages is complete, thereplaced log pages may be added to the free page pool. Note that in someembodiments, there may not be any explicit chaining of log pages usingany pointers. Instead, the sequence of log pages may be implicitlydetermined by the flush numbers on those pages. Whenever multiple copiesof a log record are found, the log record present in the log page withhighest flush number may be considered to be valid and the others may beconsidered to be obsolete.

In some embodiments, e.g., because the granularity of space managedwithin a data zone (sector) may be different from the granularityoutside the data zone (storage page), there may be some fragmentation.In some embodiments, to keep this fragmentation under control, thesystem may keep track of the number of sectors used by each data page,may preferentially allocate from almost-full data pages, and maypreferentially garbage collect almost-empty data pages (which mayrequire moving data to a new location if it is still relevant). Notethat pages allocated to a segment may in some embodiments be repurposedamong the three zones. For example, when a page that was allocated to asegment is freed, it may remain associated with that segment for someperiod of time and may subsequently be used in any of the three zones ofthat segment. The sector header of every sector may indicate the zone towhich the sector belongs. Once all sectors in a page are free, the pagemay be returned to a common free storage page pool that is shared acrosszones. This free storage page sharing may in some embodiments reduce (oravoid) fragmentation.

In some embodiments, the distributed storage systems described hereinmay maintain various data structures in memory. For example, for eachuser page present in a segment, a user page table may store a bitindicating whether or not this user page is “cleared” (i.e., whether itincludes all zeroes), the LSN of the latest log record from the cold logzone for the page, and an array/list of locations of all log recordsfrom the hot log zone for page. For each log record, the user page tablemay store the sector number, the offset of the log record within thatsector, the number of sectors to read within that log page, the sectornumber of a second log page (if the log record spans log pages), and thenumber of sectors to read within that log page. In some embodiments, theuser page table may also store the LSNs of every log record from thecold log zone and/or an array of sector numbers for the payload of thelatest AULR if it is in the cold log zone.

In some embodiments of the distributed storage systems described herein,an LSN index may be stored in memory. An LSN index may map LSNs to logpages within the cold log zone. Given that log records in cold log zoneare sorted, it may be to include one entry per log page. However, insome embodiments, every non-obsolete LSN may be stored in the index andmapped to the corresponding sector numbers, offsets, and numbers ofsectors for each log record.

In some embodiments of the distributed storage systems described herein,a log page table may be stored in memory, and the log page table may beused during garbage collection of the cold log zone. For example, thelog page table may identify which log records are obsolete (e.g., whichlog records can be garbage collected) and how much free space isavailable on each log page.

In the storage systems described herein, an extent may be a logicalconcept representing a highly durable unit of storage that can becombined with other extents (either concatenated or striped) torepresent a volume. Each extent may be made durable by membership in asingle protection group. An extent may provide an LSN-type read/writeinterface for a contiguous byte sub-range having a fixed size that isdefined at creation. Read/write operations to an extent may be mappedinto one or more appropriate segment read/write operations by thecontaining protection group. As used herein, the term “volume extent”may refer to an extent that is used to represent a specific sub-range ofbytes within a volume.

As noted above, a volume may consist of multiple extents, eachrepresented by a protection group consisting of one or more segments. Insome embodiments, log records directed to different extents may haveinterleaved LSNs. For changes to the volume to be durable up to aparticular LSN it may be necessary for all log records up to that LSN tobe durable, regardless of the extent to which they belong. In someembodiments, the client may keep track of outstanding log records thathave not yet been made durable, and once all ULRs up to a specific LSNare made durable, it may send a Volume Durable LSN (VDL) message to oneof the protection groups in the volume. The VDL may be written to allsynchronous mirror segments (i.e. group members) for the protectiongroup. This is sometimes referred to as an “Unconditional VDL” and itmay be periodically persisted to various segments (or more specifically,to various protection groups) along with write activity happening on thesegments. In some embodiments, the Unconditional VDL may be stored inlog sector headers.

In various embodiments, the operations that may be performed on asegment may include writing a DULR or AULR received from a client (whichmay involve writing the DULR or AULR to the tail of the hot log zone andthen updating the user page table), reading a cold user page (which mayinvolve locating the data sectors of the user page and returning themwithout needing to apply any additional DULRs), reading a hot user page(which may involve locating the data sectors of the most recent AULR forthe user page and apply any subsequent DULRs to the user page beforereturning it), replacing DULRs with AULRs (which may involve coalescingDULRs for a user page to create an AULR that replaces the last DULR thatwas applied), manipulating the log records, etc. As described hereincoalescing is the process of applying DULRs to an earlier version of auser page to create a later version of the user page. Coalescing a userpage may help reduce read latency because (until another DULR iswritten) all DULRs written prior to coalescing may not need to be readand applied on demand. It may also help reclaim storage space by makingold AULRs and DULRs obsolete (provided there is no snapshot requiringthe log records to be present). In some embodiments, a coalescingoperation may include locating a most recent AULR and applying anysubsequent DULRs in sequence without skipping any of the DULRs. As notedabove, in some embodiments, coalescing may not be performed within thehot log zone. Instead, it may be performed within the cold log zone. Insome embodiments, coalescing may also be performed as log records arecopied from the hot log zone to the cold log zone.

In some embodiments, the decision to coalesce a user page may betriggered by the size of the pending DULR chain for the page (e.g., ifthe length of the DULR chain exceeds a pre-defined threshold for acoalescing operation, according to a system-wide, application-specificor client-specified policy)), or by the user page being read by aclient.

FIG. 7 is a block diagram illustrating an example configuration of adatabase volume 710, according to one embodiment. In this example, datacorresponding to each of various address ranges 715 (shown as addressranges 715 a-715 e) is stored as different segments 745 (shown assegments 745 a-745 n). More specifically, data corresponding to each ofvarious address ranges 715 may be organized into different extents(shown as extents 725 a-725 b, and extents 735 a-735 h), and variousones of these extents may be included in different protection groups 730(shown as 730 a-730 f), with or without striping (such as that shown asstripe set 720 a and stripe set 720 b). In this example, protectiongroup 1 illustrates the use of erasure coding. In this example,protection groups 2 and 3 and protection groups 6 and 7 representmirrored data sets of each other, while protection group 4 represents asingle-instance (non-redundant) data set. In this example, protectiongroup 8 represents a multi-tier protection group that combines otherprotection groups (e.g., this may represent a multi-region protectiongroup). In this example, stripe set 1 (720 a) and stripe set 2 (720 b)illustrates how extents (e.g., extents 725 a and 725 b) may be stripedinto a volume, in some embodiments.

More specifically, in this example, protection group 1 (730 a) includesextents a-c (735 a-735 c), which include data from ranges 1-3 (715 a-715c), respectively, and these extents are mapped to segments 1-4 (745a-745 d). Protection group 2 (730 b) includes extent d (735 d), whichincludes data striped from range 4 (715 d), and this extent is mapped tosegments 5-7 (745 e-745 g). Similarly, protection group 3 (730 c)includes extent e (735 e), which includes data striped from range 4 (715d), and is mapped to segments 8-9 (745 h-745 i); and protection group 4(730 d) includes extent f (735 f), which includes data striped fromrange 4 (715 d), and is mapped to segment 10 (745 j). In this example,protection group 6 (730 e) includes extent g (735 g), which includesdata striped from range 5 (715 e), and is mapped to segments 11-12 (745k-745 l); and protection group 7 (730 f) includes extent h (735 h),which also includes data striped from range 5 (715 e), and is mapped tosegments 13-14 (745 m-745 n).

Please note that the striping, erasure coding, and other storage schemesfor the database volume apply to the user data space of the databasevolume, not the log records pertaining to the volume. Log records aresegmented across protection groups according to the partition of thevolume maintained at the protection group. For example, log recordsindicating updates to the user data striped from range 5 maintained inPG 6, pertain to the user data in PG 6.

The distributed storage service and database service discussed in FIGS.2 through 7 above represent some of the various different interactionsbetween a database system and a log-structured distributed storagesystem using a single log sequence number space. FIG. 8 is a high-levelflowchart illustrating a technique for implementing log-structureddistributed storage using a single log sequence number space, accordingto some embodiments. Various different storage clients, such asdifferent database systems, may maintain a data volume in alog-structured distributed storage system.

As indicated at 810, a log for a data volume may be maintained at alog-structured distributed storage system, in various embodiments. Adata volume may be user data stored for a storage client, such as adatabase system(e.g., database tables, records or entries, metadatadescribing the database, such as data dictionaries, transaction tables,and/or index structures). The user data space may be the logicalarrangement of the user data in the volume, which may be partitionedamong protection groups. For example, as FIG. 7 illustrates, differentbyte or page ranges of the data volume may be maintained by differentprotection groups. The log maintained for the data volume may indicatechanges or updates to the data and/or metadata for the data volume. Forexample, in some embodiments, the data volume may be composed of datapages, and the changes or updates to the data volume may be log recordsindicating a change or update to a particular data page in the datavolume.

As discussed above, the changes or updates to the log may be log recordsthat are assigned a log sequence number according to a log sequencenumber space for the data volume, in some embodiments. For instance,when a log record is generated, a log sequence number from the logsequence number space may be selected. The log sequence number space maybe, in various embodiments, monotonically increasing. The log sequencenumber selected for a log record may not be tightly packed or contiguouswith the prior log sequence number in the log sequence number space forthe data volume. For instance, the sequence of log records in the numberspace in order may be sparse, such as 22010, 22027, 22030, 22031, 22039,etc . . . . In some other embodiments, the log sequence numbers may becontiguous, such as 22010, 22011, 22012, 22013, etc . . . .

Also indicated at 810, the log may be segmented across multipleprotection groups according to the partitioning of user data for thedata volume. Segmenting the log may occur by individual log record, andas such, the term segment as applied to the log may not be construed asto performing striping of log records or assigning ranges of log recordsto protection groups. Instead log records pertaining to a specificpartition of user data may be co-located at the same protection group.Thus the number of log records, or ranges of LSNs within a segment ofthe log for the data volume maintained at a protection group may widelyvary. Protection groups, as discussed above with regard to FIGS. 2-7,may include one or more storage nodes providing redundant storage for agiven segment of the log.

As indicated at 820, an update to the log for the data volume may bereceived, in some embodiments. The update to the log may be part of anupdate to the data, such as set of updates or changes initiated by auser or system transaction performed by the storage client. A log recordmay be generated to indicate the update and assigned a log sequencenumber as indicated above.

In various embodiments, in response to receiving the update, aprotection group of the multiple protection groups maintaining the logfor the data volume may be determined, according to which partition ofuser data space the update pertains, as indicated at 830. As notedabove, mapping information may be maintained, at a data node or otherapplication (e.g., client-side storage driver 425 described above withregard to FIG. 4) that provides locations of different partitions of theuser data space among the protection groups. For example, if the updatepertains to data page 12111, then the mapping information may indicatethat data page 12111 is maintained at protection group A. The differentstorage nodes that are members of the protection group may also beincluded in the mapping information such that log records may be sent tomultiple storage nodes in the protection group, in some embodiments, inorder to satisfy a write quorum requirement for the log record.

In some embodiments, as indicated at 840, metadata to be included withthe log record indicating the update may be generated that indicates aprevious log sequence number of a log record maintained at thedetermined protection group. This indication may be a location, address,pointer, or simply the LSN value. In some embodiments, as storage nodesin a protection group receive log records with the included metadata,in-memory data structures, such as indexes, lists, or sequences may beupdated to include the new log record as well as its position relativeto the previous log record maintained at the protection group. Overtime, the sequence of log records maintained at a protection group maybe determined, as each log record may point to a different log record.This sequence may be a recovery sequence, which may be used to determinea completion point of the protection group in order to perform recoveryfor a database. FIG. 10, discussed below, provides further examples ofhow the generated metadata may be used to perform recovery.

Additional metadata may be included with the log record, in at leastsome embodiments, that indicates the previous log sequence number in thelog sequence number space for the data volume. For example, as notedabove, log sequence numbers may be sparse. For sparse log sequencenumbers it may not be apparent based on the log sequence number alonewhether or not another log record with a sequence number has beencommitted. For instance, LSN 22245 and LSN 22249 may be contiguous in asparse ordering of LSNs, or there may be another one or more log recordswith LSNs in between. If metadata were included with the log record ofLSN 22249 that indicates that log record LSN 22245 is the previous logrecord in the sequence, then it may be determined that all log recordsbetween LSN 22249 and LSN 22245 have been committed. FIG. 9, discussedbelow, provides further details of log recovery where suchdeterminations may be performed.

As indicated at 850, the log record may be sent to the protection groupin order to be committed to the log, in some embodiments. The protectiongroup, as previously discussed, may in various embodiments beimplemented by one or more storage nodes. By sending the log record tothe protection group, the log record may be sent to different ones (orall of) the storage nodes in a protection group. A protection group mayimplement policies or requirements in order for a log record to beconsidered durably or persistently stored. For example, in someembodiments, a write quorum requirement may determine that a certainnumber of storage nodes (e.g., 3/3 nodes, ⅗ nodes, ⅔ nodes inavailability zone A and ⅔ nodes in availability zone B, etc . . . ) in aprotection group may have to receive and persist a log record for it tobe persistently stored. Write quorum requirements or other protectiongroup policies for durability or persistence may vary based on storageclient requirements.

Based, at least in part, on acknowledgments received from storage nodesin the protection group, the log record may be identified as committedto the log for the data volume, as indicated at 860. For example, insome embodiments, a database, like database system 400 described abovewith regard to FIG. 4, may determine whether acknowledgments receivedfrom storage nodes in a protection group meet a write quorum requirementfor the log record. Identifying the log record as eligible fordurability may include acknowledging to a client of the database, suchas a user, system, or device, that the log record has been made durable,like the database query response 517 discussed above with regard to FIG.5. Log records that are eligible for durability may be identified asdurably persisted at the protection group. Upon a determination that logrecords in the log sequence number space prior to the eligible logrecord are also durable, the log record that is eligible for durabilitymay be considered durability committed to the log (e.g., to be persistedin the event of recovery).

One advantage of log-structured distributed storage is the updates tothe log for the data volume are made consistent across the log for theentire data volume without communicating the log record to otherprotection groups in the distributed storage system, reducing latencyfor committing updates to the log. Resolving differences among logrecords committed at different protection groups may be resolved atrecovery time instead. Upon a failure of the storage client (e.g.,database head node, such as database engine head node 420 in FIG. 4,head node 520 in FIG. 5), a recovery point for the entire log may bedetermined that maintains consistent view of the data volume. A storageclient, such as a database system, engine, head node, or other databasecomponent may perform the techniques illustrated below. Likewise arecovery module or service, such as may be implemented by thelog-structured distributed storage system may also perform the variousdiscussed techniques. FIG. 9 is a high-level flowchart illustrating atechnique for recovery in log-structured distributed storage using asingle log sequence number space, according to some embodiments.

As indicated at 910, a failure of a storage client may be recoveredfrom, in various embodiments. Failure may include a system or powerfailure, of a computing system or device, such as computing system 1200in FIG. 12. In some embodiments, failure may be a process implementing avirtual instance implementing the database that is frozen or restarted.The same system or virtualized instance may recover from the databasefailure, or a new virtualized instance created and/or a new computingsystem started as the database.

Upon recovery from the failure, candidate log records for recovery maybe requested from each of multiple protection groups storing the log forthe data volume may be requested, as indicated at 920. The requests forcandidate log records may be sent to one or more storage nodes in theprotection group. Further coordination or evaluations may be made at astorage client node or recovery service performing recovery if, forinstance, candidate records from different storage nodes in the sameprotection group are sent. As discussed above, the log for the datavolume may be segmented among the protection groups according to apartition of the user data. Each protection group may maintain a segmentof log records, a log specific to the protection group, from whichvolume-wide recovery may be performed. Candidate log records may be logrecords that indicate possible recovery points for the log for the datavolume. FIG. 10 is a high-level flowchart illustrating techniques forselecting candidate log records for log recovery, according to someembodiments.

As indicated at 1010, the request for a candidate log records may bereceived at a protection group. Based, at least in part, on metadataindicating a previous log sequence number for each of the log recordsmaintained at the protection group, different log records may beselected according to an identified position in recovery sequence forthe protection group. A recovery sequence for the protection group mayidentify an ordering of log records maintained at the protection group.As log records may be persisted a protection group based on thepartition of user data maintained at the protection group, the logsequence number space for the data volume, may be insufficient todiscern whether a particular log record is missing or notcommitted/persisted at a particular protection group. FIG. 11 is a blockdiagram illustrating a log segmented according to partitions of userdata space in log-structured distributed storage, according to someembodiments. For example, protection group 1150 a maintains LSN 1108 andLSN 1104. It may not be discernible based on these LSNs that no otherlog records with LSNs between these log records are to be persisted atprotection group 1150 a. Thus, metadata for the log records, such asmetadata generated above at element 840 in FIG. 8, may be used in theevaluation to select candidate log records.

The metadata for log records may, in various embodiments, be used togenerate an in-memory data structure at a storage node indicating linksbetween the log records of the log segment maintained at the respectiveprotection group. For instance, FIG. 11 illustrates previous logsequence number indicators 1152 a. LSN 1108 back links to LSN 1104,which in turn back links to LSN 1103 and to LSN 1102. Such metadata mayestablish the recovery sequence for the protection group. Based on therecovery sequence, in some embodiments, a completion point for therespective protection group may be determined. A completion point may bedetermined by traversing the links between the log records in thein-memory data structure to locate the first break or gap in thesequence. For protection group, 1150 the first break is illustratedafter LSN 1108. The completion point for the protection group may beidentified as the LSN prior to the break, LSN 1108. Similar techniquesbased on previous log sequence number indicators 1152 b may yield acompletion point for protection group 1150 b as LSN 1117, and based onprevious log sequence number indicators 1152 c may yield a completionpoint for protection group 1150 c as LSN 1115.

Upon determining a completion point for a protection group, LSNs below aprotection group may be considered eligible candidate log records forrecovery. In at least some embodiments, all eligible log records may besent as candidate log records for recovery. However, in someembodiments, eligible log records may be filtered down to a subset ofeligible log records. In some embodiments, a protection group may beaware of an LSN value that is complete across all of the protectiongroups. For example, as discussed above with regard to FIGS. 2-5, astorage client head node may send an unconditional volume LSN indicatorto protection groups, indicating an LSN value that is complete acrossall protection groups. Using this volume-wide completion point as afloor value, eligible log records equal to or below the floor LSN valuemay filtered out of the candidate log records. Other types of filteringmay be performed alone, or in combination with the floor value. Forexample, in some embodiments, as indicated at 1030, selected log records(i.e. eligible log records) may be removed that do not indicate acomplete system transaction. For instance, in some embodiments, logrecords that are the last log record associated with a systemtransaction may be marked with a consistency point indicator. Theseconsistency point LSNs may be, in some embodiments, the only eligiblelog records for candidate log records. Thus, as indicated at 1030, thoseeligible log records not indicated as consistency points may be removedfrom the candidate log records. Once the selected (i.e., eligible) logrecords have been identified, and in some embodiments filtered down, theselected log records may be sent as the candidate log records for theprotection group for log recovery, as indicated at 1040.

Turning back to FIG. 9, upon receipt of the candidate log records ormetadata for the candidate log records from each of the protectiongroups maintaining the log for the data volume (e.g., a listing of LSNvalues for the candidate log records, pointers or indicators to priorlog records at the protection group and/or in the log sequence numberspace for the entire volume), the candidate log records may be evaluatedto identify a recovery point in the log for the data volume according tothe log sequence number space, as indicated at 930. A recovery point forthe log is a completion point of the log that provides a consistent viewof the data volume. The candidate log records may each indicate LSNvalues that may possibly indicate the recovery point. Similar todetermining the completion point for a protection group, evaluating logrecords to determine a recovery point may begin by following the logsequence numbers of the candidate log records in order to determine afirst break (or highest LSN)in the log sequence number space. In someembodiments, contiguously assigned log records may inherently determinethe recovery point. For example, if the candidate log records receivedfrom protection groups 1150 a, 1150 b, and 1150 c include LSN 1108, LSN1109, LSN 1110, LSN 1111, LSN 1112, LSN 1113, LSN 1114, LSN 1115, andLSN 1117, the first break may be between LSN 1115 and LSN 1117. Thedetermined recovery point may be identified as LSN 1115, as it is knownthat a log record between LSN 1115 and LSN 1117 since log sequencenumbers are contiguously assigned. However, as discussed above withregard to FIG. 8, in some embodiments, log sequence numbers assignedfrom the log sequence number space for the data volume may be sparse.Metadata may be included with candidate log records that indicatesprevious log sequence numbers in the log sequence number space in thedata volume, in some embodiments. As the storage client has recoveredfrom failure, this information may not be available to the storageclient (or other recovery module) without the metadata. FIG. 11illustrates such metadata as log sequence number space previous LSNindicators 1140, illustrated by the dashed lines. Consider, for example,the same scenario of candidate log records, but assume that the LSNs aresparsely assigned. A pointer, indicator, other mechanism for mayindicate point in the log sequence number space where the gap or breakoccurs as it may not be known whether a break between two sparselyassigned LSNs occurs based on the LSNs alone.

As indicated at 940, a view of the data volume may be made available foraccess requests consistent with the recovery point in the log for thedata volume, in various embodiments. For example, when servicing accessrequests, such as read requests for the data volume, log records mayonly be applied up to the recovery point (in addition to any new logrecords generated after recovery which may also be applied). Such a viewof the data volume may be made available at the protection groupsservicing the access requests, in some embodiments, by applying the logrecords locally at the protection group to generate the requested data.In at least some embodiments, the recovery point may indicate atruncation point or range in the log, excluding log records with LSNswithin the truncated range from the view of the data volume. Thistruncation point (along with the range of excluded log records) may besent to and persisted at each of the protection groups so that when, asdiscussed in the example above of applying log records at the storagenodes, log records are applied at the protection groups, the truncatedlog records may not be applied.

FIG. 12 is a block diagram illustrating a computer system configured toimplement at least a portion of the database systems described herein,according to various embodiments. For example, computer system 1200 maybe configured to implement a database engine head node of a databasetier, or one of a plurality of storage nodes of a separate distributedstorage system that stores databases and associated metadata on behalfof clients of the database tier, in different embodiments. Computersystem 1200 may be any of various types of devices, including, but notlimited to, a personal computer system, desktop computer, laptop ornotebook computer, mainframe computer system, handheld computer,workstation, network computer, a consumer device, application server,storage device, telephone, mobile telephone, or in general any type ofcomputing device.

Computer system 1200 includes one or more processors 1210 (any of whichmay include multiple cores, which may be single or multi-threaded)coupled to a system memory 1220 via an input/output (I/O) interface1230. Computer system 1200 further includes a network interface 1240coupled to I/O interface 1230. In various embodiments, computer system1200 may be a uniprocessor system including one processor 1210, or amultiprocessor system including several processors 1210 (e.g., two,four, eight, or another suitable number). Processors 1210 may be anysuitable processors capable of executing instructions. For example, invarious embodiments, processors 1210 may be general-purpose or embeddedprocessors implementing any of a variety of instruction setarchitectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, orany other suitable ISA. In multiprocessor systems, each of processors1210 may commonly, but not necessarily, implement the same ISA. Thecomputer system 1200 also includes one or more network communicationdevices (e.g., network interface 1240) for communicating with othersystems and/or components over a communications network (e.g. Internet,LAN, etc.). For example, a client application executing on system 1200may use network interface 1240 to communicate with a server applicationexecuting on a single server or on a cluster of servers that implementone or more of the components of the database systems described herein.In another example, an instance of a server application executing oncomputer system 1200 may use network interface 1240 to communicate withother instances of the server application (or another serverapplication) that may be implemented on other computer systems (e.g.,computer systems 1290).

In the illustrated embodiment, computer system 1200 also includes one ormore persistent storage devices 1260 and/or one or more I/O devices1280. In various embodiments, persistent storage devices 1260 maycorrespond to disk drives, tape drives, solid state memory, other massstorage devices, or any other persistent storage device. Computer system1200 (or a distributed application or operating system operatingthereon) may store instructions and/or data in persistent storagedevices 1260, as desired, and may retrieve the stored instruction and/ordata as needed. For example, in some embodiments, computer system 1200may host a storage system server node, and persistent storage 1260 mayinclude the SSDs attached to that server node.

Computer system 1200 includes one or more system memories 1220 that areconfigured to store instructions and data accessible by processor(s)1210. In various embodiments, system memories 1220 may be implementedusing any suitable memory technology, (e.g., one or more of cache,static random access memory (SRAM), DRAM, RDRAM, EDO RAM, DDR 10 RAM,synchronous dynamic RAM (SDRAM), Rambus RAM, EEPROM,non-volatile/Flash-type memory, or any other type of memory). Systemmemory 1220 may contain program instructions 1225 that are executable byprocessor(s) 1210 to implement the methods and techniques describedherein. In various embodiments, program instructions 1225 may be encodedin platform native binary, any interpreted language such as Java™byte-code, or in any other language such as C/C++, Java™, etc., or inany combination thereof. For example, in the illustrated embodiment,program instructions 1225 include program instructions executable toimplement the functionality of a database engine head node of a databasetier, or one of a plurality of storage nodes of a separate distributedstorage system that stores databases and associated metadata on behalfof clients of the database tier, in different embodiments. In someembodiments, program instructions 1225 may implement multiple separateclients, server nodes, and/or other components.

In some embodiments, program instructions 1225 may include instructionsexecutable to implement an operating system (not shown), which may beany of various operating systems, such as UNIX, LINUX, Solaris™, MacOS™,Windows™, etc. Any or all of program instructions 1225 may be providedas a computer program product, or software, that may include anon-transitory computer-readable storage medium having stored thereoninstructions, which may be used to program a computer system (or otherelectronic devices) to perform a process according to variousembodiments. A non-transitory computer-readable storage medium mayinclude any mechanism for storing information in a form (e.g., software,processing application) readable by a machine (e.g., a computer).Generally speaking, a non-transitory computer-accessible medium mayinclude computer-readable storage media or memory media such as magneticor optical media, e.g., disk or DVD/CD-ROM coupled to computer system1200 via I/O interface 1230. A non-transitory computer-readable storagemedium may also include any volatile or non-volatile media such as RAM(e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc., that may beincluded in some embodiments of computer system 1200 as system memory1220 or another type of memory. In other embodiments, programinstructions may be communicated using optical, acoustical or other formof propagated signal (e.g., carrier waves, infrared signals, digitalsignals, etc.) conveyed via a communication medium such as a networkand/or a wireless link, such as may be implemented via network interface1240.

In some embodiments, system memory 1220 may include data store 1245,which may be configured as described herein. For example, theinformation described herein as being stored by the database tier (e.g.,on a database engine head node), such as a transaction log, an undo log,cached page data, or other information used in performing the functionsof the database tiers described herein may be stored in data store 1245or in another portion of system memory 1220 on one or more nodes, inpersistent storage 1260, and/or on one or more remote storage devices1270, at different times and in various embodiments. Similarly, theinformation described herein as being stored by the storage tier (e.g.,redo log records, coalesced data pages, and/or other information used inperforming the functions of the distributed storage systems describedherein) may be stored in data store 1245 or in another portion of systemmemory 1220 on one or more nodes, in persistent storage 1260, and/or onone or more remote storage devices 1270, at different times and invarious embodiments. In general, system memory 1220 (e.g., data store1245 within system memory 1220), persistent storage 1260, and/or remotestorage 1270 may store data blocks, replicas of data blocks, metadataassociated with data blocks and/or their state, database configurationinformation, and/or any other information usable in implementing themethods and techniques described herein.

In one embodiment, I/O interface 1230 may be configured to coordinateI/O traffic between processor 1210, system memory 1220 and anyperipheral devices in the system, including through network interface1240 or other peripheral interfaces. In some embodiments, I/O interface1230 may perform any necessary protocol, timing or other datatransformations to convert data signals from one component (e.g., systemmemory 1220) into a format suitable for use by another component (e.g.,processor 1210). In some embodiments, I/O interface 1230 may includesupport for devices attached through various types of peripheral buses,such as a variant of the Peripheral Component Interconnect (PCI) busstandard or the Universal Serial Bus (USB) standard, for example. Insome embodiments, the function of I/O interface 1230 may be split intotwo or more separate components, such as a north bridge and a southbridge, for example. Also, in some embodiments, some or all of thefunctionality of I/O interface 1230, such as an interface to systemmemory 1220, may be incorporated directly into processor 1210.

Network interface 1240 may be configured to allow data to be exchangedbetween computer system 1200 and other devices attached to a network,such as other computer systems 1290 (which may implement one or morestorage system server nodes, database engine head nodes, and/or clientsof the database systems described herein), for example. In addition,network interface 1240 may be configured to allow communication betweencomputer system 1200 and various I/O devices 1250 and/or remote storage1270. Input/output devices 1250 may, in some embodiments, include one ormore display terminals, keyboards, keypads, touchpads, scanning devices,voice or optical recognition devices, or any other devices suitable forentering or retrieving data by one or more computer systems 1200.Multiple input/output devices 1250 may be present in computer system1200 or may be distributed on various nodes of a distributed system thatincludes computer system 1200. In some embodiments, similar input/outputdevices may be separate from computer system 1200 and may interact withone or more nodes of a distributed system that includes computer system1200 through a wired or wireless connection, such as over networkinterface 1240. Network interface 1240 may commonly support one or morewireless networking protocols (e.g., Wi-Fi/IEEE 802.11, or anotherwireless networking standard). However, in various embodiments, networkinterface 1240 may support communication via any suitable wired orwireless general data networks, such as other types of Ethernetnetworks, for example. Additionally, network interface 1240 may supportcommunication via telecommunications/telephony networks such as analogvoice networks or digital fiber communications networks, via storagearea networks such as Fibre Channel SANs, or via any other suitable typeof network and/or protocol. In various embodiments, computer system 1200may include more, fewer, or different components than those illustratedin FIG. 12 (e.g., displays, video cards, audio cards, peripheraldevices, other network interfaces such as an ATM interface, an Ethernetinterface, a Frame Relay interface, etc.)

It is noted that any of the distributed system embodiments describedherein, or any of their components, may be implemented as one or morenetwork-based services. For example, a database engine head node withinthe database tier of a database system may present database servicesand/or other types of data storage services that employ the distributedstorage systems described herein to clients as network-based services.In some embodiments, a network-based service may be implemented by asoftware and/or hardware system designed to support interoperablemachine-to-machine interaction over a network. A network-based servicemay have an interface described in a machine-processable format, such asthe Web Services Description Language (WSDL). Other systems may interactwith the network-based service in a manner prescribed by the descriptionof the network-based service's interface. For example, the network-basedservice may define various operations that other systems may invoke, andmay define a particular application programming interface (API) to whichother systems may be expected to conform when requesting the variousoperations.

In various embodiments, a network-based service may be requested orinvoked through the use of a message that includes parameters and/ordata associated with the network-based services request. Such a messagemay be formatted according to a particular markup language such asExtensible Markup Language (XML), and/or may be encapsulated using aprotocol such as Simple Object Access Protocol (SOAP). To perform anetwork-based services request, a network-based services client mayassemble a message including the request and convey the message to anaddressable endpoint (e.g., a Uniform Resource Locator (URL))corresponding to the network-based service, using an Internet-basedapplication layer transfer protocol such as Hypertext Transfer Protocol(HTTP).

In some embodiments, network-based services may be implemented usingRepresentational State Transfer (“RESTful”) techniques rather thanmessage-based techniques. For example, a network-based serviceimplemented according to a RESTful technique may be invoked throughparameters included within an HTTP method such as PUT, GET, or DELETE,rather than encapsulated within a SOAP message.

The various methods as illustrated in the figures and described hereinrepresent example embodiments of methods. The methods may be implementedmanually, in software, in hardware, or in a combination thereof. Theorder of any method may be changed, and various elements may be added,reordered, combined, omitted, modified, etc.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications may be made as wouldbecome apparent to those skilled in the art once the above disclosure isfully appreciated. It is intended that the following claims beinterpreted to embrace all such modifications and changes and,accordingly, the above description to be regarded in an illustrativerather than a restrictive sense.

1-21. (canceled)
 22. A system, comprising: a memory to store programinstructions which, if performed by at least one processor, cause the atleast one processor to perform a method to at least: detect a failure ofa storage client of a distributed data store that stores a log for adata volume, wherein the log is segmented across a plurality ofdifferent groups of storage nodes that store different partitions of thedata volume, and wherein the log comprises a plurality of log recordsassigned respective log sequence numbers according to a log sequencenumber space for the data volume; and obtain from each of the pluralityof groups one or more log records that indicate possible recovery pointsfor the log; evaluate the assigned log sequence numbers for the obtainedlog records to identify a recovery point in the log for the data volumeaccording to the log sequence number space; and make a view of the datavolume available for access requests consistent with the recovery pointin the log for the data volume.
 23. The system of claim 22, wherein toobtain from each of the plurality of groups one or more log records thatindicate possible recovery points for the log, comprises send a requestfor the one or more log records that indicate the possible recoverypoints for the logs to at least one storage node in each of the groupsof storage nodes; wherein the method further comprises: for each of thegroups of storage nodes: receive the request at the at least one storagenode; based, at least in part, on metadata indicating a previous logsequence number for log records maintained at the group of storagenodes, select, by the at least one storage node, one or more candidatelog records of the log records maintained at the group of storage nodesaccording to a recovery sequence for the log records maintained at thegroup of storage nodes; and send, from the at least one storage node,respective metadata for the one or more candidate log records inresponse to the request for the one or more log records.
 24. The systemof claim 23, wherein the method further comprises: for each of thegroups of storage nodes, remove, by the at least one storage node, atleast one of the candidate log records that do not indicate completionof a system transaction.
 25. The system of claim 23, wherein to selectthe one or more candidate log records of the log records comprisestraverse one or more links between the log records in the metadata tolocate a first break in the recovery sequence.
 26. The system of claim23, wherein the selection of the one or more candidate log records isbased, at least in part, on a volume-wide recovery point in themetadata.
 27. The system of claim 22, wherein to evaluate the assignedlog sequence numbers for the obtained log records to identify therecovery point comprises determine a break in the log sequence numberspace for the obtained log records.
 28. The system of claim 22, whereinthe memory and the at least one processor are implemented as part of arecovery service for a network-based database service.
 29. A method,comprising: detecting a failure of a storage client of a distributeddata store that stores a log for a data volume, wherein the log issegmented across a plurality of different groups of storage nodes thatstore different partitions of the data volume, and wherein the logcomprises a plurality of log records assigned respective log sequencenumbers according to a log sequence number space for the data volume;and obtaining from each of the plurality of groups one or more logrecords that indicate possible recovery points for the log; evaluatingthe assigned log sequence numbers for the obtained log records toidentify a recovery point in the log for the data volume according tothe log sequence number space; and making a view of the data volumeavailable for access requests consistent with the recovery point in thelog for the data volume.
 30. The method of claim 29, wherein theobtaining from each of the plurality of groups one or more log recordsthat indicate possible recovery points for the log, comprises sending arequest for the one or more log records that indicate the possiblerecovery points for the logs to at least one storage node in each of thegroups of storage nodes; wherein the method further comprises: for eachof the groups of storage nodes: receiving the request at the at leastone storage node; based, at least in part, on metadata indicating aprevious log sequence number for log records maintained at the group ofstorage nodes, selecting, by the at least one storage node, one or morecandidate log records of the log records maintained at the group ofstorage nodes according to a recovery sequence for the log recordsmaintained at the group of storage nodes; and sending, from the at leastone storage node, respective metadata for the one or more candidate logrecords in response to the request for the one or more log records. 31.The method of claim 30, further comprising: for each of the groups ofstorage nodes, removing, by the at least one storage node, at least oneof the candidate log records that do not indicate completion of a systemtransaction.
 32. The method of claim 30, wherein the selecting the oneor more candidate log records of the log records comprises traversingone or more links between the log records in the metadata to locate afirst break in the recovery sequence.
 34. The method of claim 30,wherein the selecting of the one or more candidate log records is based,at least in part, on a volume-wide recovery point in the metadata. 34.The method of claim 29, wherein the evaluating the assigned log sequencenumbers for the obtained log records to identify the recovery pointcomprises determining a break in the log sequence number space for theobtained log records.
 35. The method of claim 29, wherein the detecting,the obtaining, the evaluating and the making are performed by a recoveryservice for a network-based database service.
 36. A non-transitory,computer-readable storage medium, comprising program instructions thatwhen executed by at least one computing device cause the at least onecomputing device to implement: detecting a failure of a storage clientof a distributed data store that stores a log for a data volume, whereinthe log is segmented across a plurality of different groups of storagenodes that store different partitions of the data volume, and whereinthe log comprises a plurality of log records assigned respective logsequence numbers according to a log sequence number space for the datavolume; and obtaining from each of the plurality of groups one or morelog records that indicate possible recovery points for the log;evaluating the assigned log sequence numbers for the obtained logrecords to identify a recovery point in the log for the data volumeaccording to the log sequence number space; and making a view of thedata volume available for access requests consistent with the recoverypoint in the log for the data volume.
 37. The non-transitory,computer-readable storage medium of claim 36, wherein, in obtaining fromeach of the plurality of groups one or more log records that indicatepossible recovery points for the log, the program instructions cause theat least one computing device to implement sending a request for the oneor more log records that indicate the possible recovery points for thelogs to at least one storage node in each of the groups of storagenodes; wherein the program instructions cause the at least one computingdevice to further implement: for each of the groups of storage nodes:receiving the request at the at least one storage node; based, at leastin part, on metadata indicating a previous log sequence number for logrecords maintained at the group of storage nodes, selecting, by the atleast one storage node, one or more candidate log records of the logrecords maintained at the group of storage nodes according to a recoverysequence for the log records maintained at the group of storage nodes;and sending, from the at least one storage node, respective metadata forthe one or more candidate log records in response to the request for theone or more log records.
 38. The non-transitory, computer-readablestorage medium of claim 37, wherein the program instructions furthercause the at least one computing device to implement: for each of thegroups of storage nodes, removing, by the at least one storage node, atleast one of the candidate log records that do not indicate completionof a system transaction.
 39. The non-transitory, computer-readablestorage medium of claim 37, wherein, in selecting the one or morecandidate log records of the log records, the program instructions causethe at least one computing device to implement traversing one or morelinks between the log records in the metadata to locate a first break inthe recovery sequence.
 40. The non-transitory, computer-readable storagemedium of claim 36, wherein, in evaluating the assigned log sequencenumbers for the obtained log records to identify the recovery point, theprogram instructions cause the at least one computing device toimplement determining a break in the log sequence number space for theobtained log records.
 41. The non-transitory, computer-readable storagemedium of claim 36, wherein the at least one computing device isimplemented as part of a recovery service for a network-based databaseservice.